Date:   Fri, 24 Feb 2023 09:33:31 +0000
From:   JeeHeng Sia <jeeheng.sia@...rfivetech.com>
To:     Andrew Jones <ajones@...tanamicro.com>
CC:     "paul.walmsley@...ive.com" <paul.walmsley@...ive.com>,
        "palmer@...belt.com" <palmer@...belt.com>,
        "aou@...s.berkeley.edu" <aou@...s.berkeley.edu>,
        "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Leyfoon Tan <leyfoon.tan@...rfivetech.com>,
        Mason Huo <mason.huo@...rfivetech.com>
Subject: RE: [PATCH v4 4/4] RISC-V: Add arch functions to support
 hibernation/suspend-to-disk



> -----Original Message-----
> From: Andrew Jones <ajones@...tanamicro.com>
> Sent: Friday, 24 February, 2023 5:00 PM
> To: JeeHeng Sia <jeeheng.sia@...rfivetech.com>
> Cc: paul.walmsley@...ive.com; palmer@...belt.com; aou@...s.berkeley.edu; linux-riscv@...ts.infradead.org; linux-
> kernel@...r.kernel.org; Leyfoon Tan <leyfoon.tan@...rfivetech.com>; Mason Huo <mason.huo@...rfivetech.com>
> Subject: Re: [PATCH v4 4/4] RISC-V: Add arch functions to support hibernation/suspend-to-disk
> 
> On Fri, Feb 24, 2023 at 02:05:43AM +0000, JeeHeng Sia wrote:
> >
> >
> > > -----Original Message-----
> > > From: Andrew Jones <ajones@...tanamicro.com>
> > > Sent: Friday, 24 February, 2023 2:07 AM
> > > To: JeeHeng Sia <jeeheng.sia@...rfivetech.com>
> > > Cc: paul.walmsley@...ive.com; palmer@...belt.com; aou@...s.berkeley.edu; linux-riscv@...ts.infradead.org; linux-
> > > kernel@...r.kernel.org; Leyfoon Tan <leyfoon.tan@...rfivetech.com>; Mason Huo <mason.huo@...rfivetech.com>
> > > Subject: Re: [PATCH v4 4/4] RISC-V: Add arch functions to support hibernation/suspend-to-disk
> > >
> > > On Tue, Feb 21, 2023 at 10:35:23AM +0800, Sia Jee Heng wrote:
> > > > Low level Arch functions were created to support hibernation.
> > > > swsusp_arch_suspend() relies code from __cpu_suspend_enter() to write
> > > > cpu state onto the stack, then calling swsusp_save() to save the memory
> > > > image.
> > > >
> > > > Arch specific hibernation header is implemented and is utilized by the
> > > > arch_hibernation_header_restore() and arch_hibernation_header_save()
> > > > functions. The arch specific hibernation header consists of satp, hartid,
> > > > and the cpu_resume address. The kernel built version is also need to be
> > > > saved into the hibernation image header to making sure only the same
> > > > kernel is restore when resume.
> > > >
> > > > swsusp_arch_resume() creates a temporary page table that covering only
> > > > the linear map. It copies the restore code to a 'safe' page, then start
> > > > to restore the memory image. Once completed, it restores the original
> > > > kernel's page table. It then calls into __hibernate_cpu_resume()
> > > > to restore the CPU context. Finally, it follows the normal hibernation
> > > > path back to the hibernation core.
> > > >
> > > > To enable hibernation/suspend to disk into RISCV, the below config
> > > > need to be enabled:
> > > > - CONFIG_ARCH_HIBERNATION_HEADER
> > > > - CONFIG_ARCH_HIBERNATION_POSSIBLE
> > > >
> > > > Signed-off-by: Sia Jee Heng <jeeheng.sia@...rfivetech.com>
> > > > Reviewed-by: Ley Foon Tan <leyfoon.tan@...rfivetech.com>
> > > > Reviewed-by: Mason Huo <mason.huo@...rfivetech.com>
> > > > ---
> > > >  arch/riscv/Kconfig                 |   7 +
> > > >  arch/riscv/include/asm/assembler.h |  20 ++
> > > >  arch/riscv/include/asm/suspend.h   |  19 ++
> > > >  arch/riscv/kernel/Makefile         |   1 +
> > > >  arch/riscv/kernel/asm-offsets.c    |   5 +
> > > >  arch/riscv/kernel/hibernate-asm.S  |  77 +++++
> > > >  arch/riscv/kernel/hibernate.c      | 447 +++++++++++++++++++++++++++++
> > > >  7 files changed, 576 insertions(+)
> > > >  create mode 100644 arch/riscv/kernel/hibernate-asm.S
> > > >  create mode 100644 arch/riscv/kernel/hibernate.c
> > > >
> > > > diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> > > > index e2b656043abf..4555848a817f 100644
> > > > --- a/arch/riscv/Kconfig
> > > > +++ b/arch/riscv/Kconfig
> > > > @@ -690,6 +690,13 @@ menu "Power management options"
> > > >
> > > >  source "kernel/power/Kconfig"
> > > >
> > > > +config ARCH_HIBERNATION_POSSIBLE
> > > > +	def_bool y
> > > > +
> > > > +config ARCH_HIBERNATION_HEADER
> > > > +	def_bool y
> > > > +	depends on HIBERNATION
> > >
> > > nit: I think this can be simplified as def_bool HIBERNATION
> > good suggestion. will change it.
> > >
> > > > +
> > > >  endmenu # "Power management options"
> > > >
> > > >  menu "CPU Power Management"
> > > > diff --git a/arch/riscv/include/asm/assembler.h b/arch/riscv/include/asm/assembler.h
> > > > index 727a97735493..68c46c0e0ea8 100644
> > > > --- a/arch/riscv/include/asm/assembler.h
> > > > +++ b/arch/riscv/include/asm/assembler.h
> > > > @@ -59,4 +59,24 @@
> > > >  		REG_L	s11, (SUSPEND_CONTEXT_REGS + PT_S11)(a0)
> > > >  	.endm
> > > >
> > > > +/*
> > > > + * copy_page - copy 1 page (4KB) of data from source to destination
> > > > + * @a0 - destination
> > > > + * @a1 - source
> > > > + */
> > > > +	.macro	copy_page a0, a1
> > > > +		lui	a2, 0x1
> > > > +		add	a2, a2, a0
> > > > +1 :
> > >     ^ please remove this space
> > can't remove it otherwise checkpatch will throws ERROR: spaces required around that ':'
> 
> Oh, right, labels in macros have this requirement.
> 
> > >
> > > > +		REG_L	t0, 0(a1)
> > > > +		REG_L	t1, SZREG(a1)
> > > > +
> > > > +		REG_S	t0, 0(a0)
> > > > +		REG_S	t1, SZREG(a0)
> > > > +
> > > > +		addi	a0, a0, 2 * SZREG
> > > > +		addi	a1, a1, 2 * SZREG
> > > > +		bne	a2, a0, 1b
> > > > +	.endm
> > > > +
> > > >  #endif	/* __ASM_ASSEMBLER_H */
> > > > diff --git a/arch/riscv/include/asm/suspend.h b/arch/riscv/include/asm/suspend.h
> > > > index 75419c5ca272..3362da56a9d8 100644
> > > > --- a/arch/riscv/include/asm/suspend.h
> > > > +++ b/arch/riscv/include/asm/suspend.h
> > > > @@ -21,6 +21,11 @@ struct suspend_context {
> > > >  #endif
> > > >  };
> > > >
> > > > +/*
> > > > + * Used by hibernation core and cleared during resume sequence
> > > > + */
> > > > +extern int in_suspend;
> > > > +
> > > >  /* Low-level CPU suspend entry function */
> > > >  int __cpu_suspend_enter(struct suspend_context *context);
> > > >
> > > > @@ -36,4 +41,18 @@ int __cpu_resume_enter(unsigned long hartid, unsigned long context);
> > > >  /* Used to save and restore the csr */
> > > >  void suspend_save_csrs(struct suspend_context *context);
> > > >  void suspend_restore_csrs(struct suspend_context *context);
> > > > +
> > > > +/* Low-level API to support hibernation */
> > > > +int swsusp_arch_suspend(void);
> > > > +int swsusp_arch_resume(void);
> > > > +int arch_hibernation_header_save(void *addr, unsigned int max_size);
> > > > +int arch_hibernation_header_restore(void *addr);
> > > > +int __hibernate_cpu_resume(void);
> > > > +
> > > > +/* Used to resume on the CPU we hibernated on */
> > > > +int hibernate_resume_nonboot_cpu_disable(void);
> > > > +
> > > > +asmlinkage void hibernate_restore_image(unsigned long resume_satp, unsigned long satp_temp,
> > > > +					unsigned long cpu_resume);
> > > > +asmlinkage int hibernate_core_restore_code(void);
> > > >  #endif
> > > > diff --git a/arch/riscv/kernel/Makefile b/arch/riscv/kernel/Makefile
> > > > index 4cf303a779ab..daab341d55e4 100644
> > > > --- a/arch/riscv/kernel/Makefile
> > > > +++ b/arch/riscv/kernel/Makefile
> > > > @@ -64,6 +64,7 @@ obj-$(CONFIG_MODULES)		+= module.o
> > > >  obj-$(CONFIG_MODULE_SECTIONS)	+= module-sections.o
> > > >
> > > >  obj-$(CONFIG_CPU_PM)		+= suspend_entry.o suspend.o
> > > > +obj-$(CONFIG_HIBERNATION)	+= hibernate.o hibernate-asm.o
> > > >
> > > >  obj-$(CONFIG_FUNCTION_TRACER)	+= mcount.o ftrace.o
> > > >  obj-$(CONFIG_DYNAMIC_FTRACE)	+= mcount-dyn.o
> > > > diff --git a/arch/riscv/kernel/asm-offsets.c b/arch/riscv/kernel/asm-offsets.c
> > > > index df9444397908..d6a75aac1d27 100644
> > > > --- a/arch/riscv/kernel/asm-offsets.c
> > > > +++ b/arch/riscv/kernel/asm-offsets.c
> > > > @@ -9,6 +9,7 @@
> > > >  #include <linux/kbuild.h>
> > > >  #include <linux/mm.h>
> > > >  #include <linux/sched.h>
> > > > +#include <linux/suspend.h>
> > > >  #include <asm/kvm_host.h>
> > > >  #include <asm/thread_info.h>
> > > >  #include <asm/ptrace.h>
> > > > @@ -116,6 +117,10 @@ void asm_offsets(void)
> > > >
> > > >  	OFFSET(SUSPEND_CONTEXT_REGS, suspend_context, regs);
> > > >
> > > > +	OFFSET(HIBERN_PBE_ADDR, pbe, address);
> > > > +	OFFSET(HIBERN_PBE_ORIG, pbe, orig_address);
> > > > +	OFFSET(HIBERN_PBE_NEXT, pbe, next);
> > > > +
> > > >  	OFFSET(KVM_ARCH_GUEST_ZERO, kvm_vcpu_arch, guest_context.zero);
> > > >  	OFFSET(KVM_ARCH_GUEST_RA, kvm_vcpu_arch, guest_context.ra);
> > > >  	OFFSET(KVM_ARCH_GUEST_SP, kvm_vcpu_arch, guest_context.sp);
> > > > diff --git a/arch/riscv/kernel/hibernate-asm.S b/arch/riscv/kernel/hibernate-asm.S
> > > > new file mode 100644
> > > > index 000000000000..846affe4dced
> > > > --- /dev/null
> > > > +++ b/arch/riscv/kernel/hibernate-asm.S
> > > > @@ -0,0 +1,77 @@
> > > > +/* SPDX-License-Identifier: GPL-2.0-only */
> > > > +/*
> > > > + * Hibernation low level support for RISCV.
> > > > + *
> > > > + * Copyright (C) 2023 StarFive Technology Co., Ltd.
> > > > + *
> > > > + * Author: Jee Heng Sia <jeeheng.sia@...rfivetech.com>
> > > > + */
> > > > +
> > > > +#include <asm/asm.h>
> > > > +#include <asm/asm-offsets.h>
> > > > +#include <asm/assembler.h>
> > > > +#include <asm/csr.h>
> > > > +
> > > > +#include <linux/linkage.h>
> > > > +
> > > > +/*
> > > > + * int __hibernate_cpu_resume(void)
> > > > + * Switch back to the hibernated image's page table prior to restoring the CPU
> > > > + * context.
> > > > + *
> > > > + * Always returns 0
> > > > + */
> > > > +ENTRY(__hibernate_cpu_resume)
> > > > +	/* switch to hibernated image's page table. */
> > > > +	csrw CSR_SATP, s0
> > > > +	sfence.vma
> > > > +
> > > > +	REG_L	a0, hibernate_cpu_context
> > > > +
> > > > +	restore_csr
> > > > +	restore_reg
> > > > +
> > > > +	/* Return zero value. */
> > > > +	add	a0, zero, zero
> > >
> > > nit: mv a0, zero
> > sure
> > >
> > > > +
> > > > +	ret
> > > > +END(__hibernate_cpu_resume)
> > > > +
> > > > +/*
> > > > + * Prepare to restore the image.
> > > > + * a0: satp of saved page tables.
> > > > + * a1: satp of temporary page tables.
> > > > + * a2: cpu_resume.
> > > > + */
> > > > +ENTRY(hibernate_restore_image)
> > > > +	mv	s0, a0
> > > > +	mv	s1, a1
> > > > +	mv	s2, a2
> > > > +	REG_L	s4, restore_pblist
> > > > +	REG_L	a1, relocated_restore_code
> > > > +
> > > > +	jalr	a1
> > > > +END(hibernate_restore_image)
> > > > +
> > > > +/*
> > > > + * The below code will be executed from a 'safe' page.
> > > > + * It first switches to the temporary page table, then starts to copy the pages
> > > > + * back to the original memory location. Finally, it jumps to __hibernate_cpu_resume()
> > > > + * to restore the CPU context.
> > > > + */
> > > > +ENTRY(hibernate_core_restore_code)
> > > > +	/* switch to temp page table. */
> > > > +	csrw satp, s1
> > > > +	sfence.vma
> > > > +.Lcopy:
> > > > +	/* The below code will restore the hibernated image. */
> > > > +	REG_L	a1, HIBERN_PBE_ADDR(s4)
> > > > +	REG_L	a0, HIBERN_PBE_ORIG(s4)
> > >
> > > Are we sure restore_pblist will never be NULL?
> > restore_pblist is a link-list, it will be null during initialization or during page clean up by hibernation core. During the initial resume
> process, the hibernation core will check the header and load the pages. If everything works correctly, the page will be linked to the
> restore_pblist and then invoke swsusp_arch_resume() else hibernation core will throws error and failed to resume from the
> hibernated image.
> 
> I know restore_pblist is a linked-list and this doesn't answer the
> question. The comment above restore_pblist says
> 
> /*
>  * List of PBEs needed for restoring the pages that were allocated before
>  * the suspend and included in the suspend image, but have also been
>  * allocated by the "resume" kernel, so their contents cannot be written
>  * directly to their "original" page frames.
>  */
> 
> which implies the pages that end up on this list are "special". My
> question is whether or not we're guaranteed to have at least one
> of these special pages. If not, we shouldn't assume s4 is non-null.
> If so, then a comment stating why that's guaranteed would be nice.
The restore_pblist will not be null otherwise swsusp_arch_resume wouldn't get invoked. you can find how the link-list are link and how it checks against validity at https://elixir.bootlin.com/linux/v6.2-rc8/source/kernel/power/snapshot.c . " A comment stating why that's guaranteed would be nice" ? Hmm, perhaps this is out of my scope but I do believe in the page validity checking in the link I shared.
> 
> > >
> > > > +
> > > > +	copy_page a0, a1
> > > > +
> > > > +	REG_L	s4, HIBERN_PBE_NEXT(s4)
> > > > +	bnez	s4, .Lcopy
> > > > +
> > > > +	jalr	s2
> > > > +END(hibernate_core_restore_code)
> > > > diff --git a/arch/riscv/kernel/hibernate.c b/arch/riscv/kernel/hibernate.c
> > > > new file mode 100644
> > > > index 000000000000..46a2f470db6e
> > > > --- /dev/null
> > > > +++ b/arch/riscv/kernel/hibernate.c
> > > > @@ -0,0 +1,447 @@
> > > > +// SPDX-License-Identifier: GPL-2.0-only
> > > > +/*
> > > > + * Hibernation support for RISCV
> > > > + *
> > > > + * Copyright (C) 2023 StarFive Technology Co., Ltd.
> > > > + *
> > > > + * Author: Jee Heng Sia <jeeheng.sia@...rfivetech.com>
> > > > + */
> > > > +
> > > > +#include <asm/barrier.h>
> > > > +#include <asm/cacheflush.h>
> > > > +#include <asm/mmu_context.h>
> > > > +#include <asm/page.h>
> > > > +#include <asm/pgalloc.h>
> > > > +#include <asm/pgtable.h>
> > > > +#include <asm/sections.h>
> > > > +#include <asm/set_memory.h>
> > > > +#include <asm/smp.h>
> > > > +#include <asm/suspend.h>
> > > > +
> > > > +#include <linux/cpu.h>
> > > > +#include <linux/memblock.h>
> > > > +#include <linux/pm.h>
> > > > +#include <linux/sched.h>
> > > > +#include <linux/suspend.h>
> > > > +#include <linux/utsname.h>
> > > > +
> > > > +/* The logical cpu number we should resume on, initialised to a non-cpu number. */
> > > > +static int sleep_cpu = -EINVAL;
> > > > +
> > > > +/* Pointer to the temporary resume page table. */
> > > > +static pgd_t *resume_pg_dir;
> > > > +
> > > > +/* CPU context to be saved. */
> > > > +struct suspend_context *hibernate_cpu_context;
> > > > +EXPORT_SYMBOL_GPL(hibernate_cpu_context);
> > > > +
> > > > +unsigned long relocated_restore_code;
> > > > +EXPORT_SYMBOL_GPL(relocated_restore_code);
> > > > +
> > > > +/**
> > > > + * struct arch_hibernate_hdr_invariants - container to store kernel build version.
> > > > + * @uts_version: to save the build number and date so that the we do not resume with
> > > > + *		a different kernel.
> > > > + */
> > > > +struct arch_hibernate_hdr_invariants {
> > > > +	char		uts_version[__NEW_UTS_LEN + 1];
> > > > +};
> > > > +
> > > > +/**
> > > > + * struct arch_hibernate_hdr - helper parameters that help us to restore the image.
> > > > + * @invariants: container to store kernel build version.
> > > > + * @hartid: to make sure same boot_cpu executes the hibernate/restore code.
> > > > + * @saved_satp: original page table used by the hibernated image.
> > > > + * @restore_cpu_addr: the kernel's image address to restore the CPU context.
> > > > + */
> > > > +static struct arch_hibernate_hdr {
> > > > +	struct arch_hibernate_hdr_invariants invariants;
> > > > +	unsigned long	hartid;
> > > > +	unsigned long	saved_satp;
> > > > +	unsigned long	restore_cpu_addr;
> > > > +} resume_hdr;
> > > > +
> > > > +static inline void arch_hdr_invariants(struct arch_hibernate_hdr_invariants *i)
> > > > +{
> > > > +	memset(i, 0, sizeof(*i));
> > > > +	memcpy(i->uts_version, init_utsname()->version, sizeof(i->uts_version));
> > > > +}
> > > > +
> > > > +/*
> > > > + * Check if the given pfn is in the 'nosave' section.
> > > > + */
> > > > +int pfn_is_nosave(unsigned long pfn)
> > > > +{
> > > > +	unsigned long nosave_begin_pfn = sym_to_pfn(&__nosave_begin);
> > > > +	unsigned long nosave_end_pfn = sym_to_pfn(&__nosave_end - 1);
> > > > +
> > > > +	return ((pfn >= nosave_begin_pfn) && (pfn <= nosave_end_pfn));
> > > > +}
> > > > +
> > > > +void notrace save_processor_state(void)
> > > > +{
> > > > +	WARN_ON(num_online_cpus() != 1);
> > > > +}
> > > > +
> > > > +void notrace restore_processor_state(void)
> > > > +{
> > > > +}
> > > > +
> > > > +/*
> > > > + * Helper parameters need to be saved to the hibernation image header.
> > > > + */
> > > > +int arch_hibernation_header_save(void *addr, unsigned int max_size)
> > > > +{
> > > > +	struct arch_hibernate_hdr *hdr = addr;
> > > > +
> > > > +	if (max_size < sizeof(*hdr))
> > > > +		return -EOVERFLOW;
> > > > +
> > > > +	arch_hdr_invariants(&hdr->invariants);
> > > > +
> > > > +	hdr->hartid = cpuid_to_hartid_map(sleep_cpu);
> > > > +	hdr->saved_satp = csr_read(CSR_SATP);
> > > > +	hdr->restore_cpu_addr = (unsigned long)__hibernate_cpu_resume;
> > > > +
> > > > +	return 0;
> > > > +}
> > > > +EXPORT_SYMBOL_GPL(arch_hibernation_header_save);
> > > > +
> > > > +/*
> > > > + * Retrieve the helper parameters from the hibernation image header.
> > > > + */
> > > > +int arch_hibernation_header_restore(void *addr)
> > > > +{
> > > > +	struct arch_hibernate_hdr_invariants invariants;
> > > > +	struct arch_hibernate_hdr *hdr = addr;
> > > > +	int ret = 0;
> > > > +
> > > > +	arch_hdr_invariants(&invariants);
> > > > +
> > > > +	if (memcmp(&hdr->invariants, &invariants, sizeof(invariants))) {
> > > > +		pr_crit("Hibernate image not generated by this kernel!\n");
> > > > +		return -EINVAL;
> > > > +	}
> > > > +
> > > > +	sleep_cpu = riscv_hartid_to_cpuid(hdr->hartid);
> > > > +	if (sleep_cpu < 0) {
> > > > +		pr_crit("Hibernated on a CPU not known to this kernel!\n");
> > > > +		sleep_cpu = -EINVAL;
> > > > +		return -EINVAL;
> > > > +	}
> > > > +
> > > > +#ifdef CONFIG_SMP
> > > > +	ret = bringup_hibernate_cpu(sleep_cpu);
> > > > +	if (ret) {
> > > > +		sleep_cpu = -EINVAL;
> > > > +		return ret;
> > > > +	}
> > > > +#endif
> > > > +	resume_hdr = *hdr;
> > > > +
> > > > +	return ret;
> > > > +}
> > > > +EXPORT_SYMBOL_GPL(arch_hibernation_header_restore);
> > > > +
> > > > +int swsusp_arch_suspend(void)
> > > > +{
> > > > +	int ret = 0;
> > > > +
> > > > +	if (__cpu_suspend_enter(hibernate_cpu_context)) {
> > > > +		sleep_cpu = smp_processor_id();
> > > > +		suspend_save_csrs(hibernate_cpu_context);
> > > > +		ret = swsusp_save();
> > > > +	} else {
> > > > +		suspend_restore_csrs(hibernate_cpu_context);
> > > > +		flush_tlb_all();
> > > > +		flush_icache_all();
> > > > +
> > > > +		/*
> > > > +		 * Tell the hibernation core that we've just restored the memory.
> > > > +		 */
> > > > +		in_suspend = 0;
> > > > +		sleep_cpu = -EINVAL;
> > > > +	}
> > > > +
> > > > +	return ret;
> > > > +}
> > > > +
> > > > +static unsigned long _temp_pgtable_map_pte(pte_t *dst_ptep, pte_t *src_ptep,
> > > > +					   unsigned long addr, pgprot_t prot)
> > > > +{
> > > > +	pte_t pte = READ_ONCE(*src_ptep);
> > > > +
> > > > +	if (pte_present(pte))
> > > > +		set_pte(dst_ptep, __pte(pte_val(pte) | pgprot_val(prot)));
> > > > +
> > > > +	return 0;
> > > > +}
> > > > +
> > > > +static unsigned long temp_pgtable_map_pte(pmd_t *dst_pmdp, pmd_t *src_pmdp,
> > > > +					  unsigned long start, unsigned long end,
> > > > +					  pgprot_t prot)
> > > > +{
> > > > +	unsigned long addr = start;
> > > > +	pte_t *src_ptep;
> > > > +	pte_t *dst_ptep;
> > > > +
> > > > +	if (pmd_none(READ_ONCE(*dst_pmdp))) {
> > > > +		dst_ptep = (pte_t *)get_safe_page(GFP_ATOMIC);
> > > > +		if (!dst_ptep)
> > > > +			return -ENOMEM;
> > > > +
> > > > +		pmd_populate_kernel(NULL, dst_pmdp, dst_ptep);
> > > > +	}
> > > > +
> > > > +	dst_ptep = pte_offset_kernel(dst_pmdp, start);
> > > > +	src_ptep = pte_offset_kernel(src_pmdp, start);
> > > > +
> > > > +	do {
> > > > +		_temp_pgtable_map_pte(dst_ptep, src_ptep, addr, prot);
> > >
> > > I think I'd rather have the body of _temp_pgtable_map_pte() here and drop
> > > the helper, because the helper does (pte_val(pte) | pgprot_val(prot))
> > > which looks strange, until seeing here that 'pte' is only the address
> > > bits, so OR'ing in new prot bits without clearing old prot bits makes
> > > sense.
> > we do not need to clear the old bits since we going to keep those bits but add new bits which are required for resume. Let's hold
> your question here but I will would like to see how Alex view it.
> 
> I confused myself a bit in my first read, so some of what I said isn't
> relevant, but I still wonder why we don't want to be more explicit about
> what prot bits are present in the end, and I still wonder why we need such
> a simple helper function which is used in exactly one place. Indeed, the
> pattern of all the other pgtable functions below is to put the set_p*
> calls directly in the loop.
I am sorry if I confused you but what I meant is that I would like to consolidate all comments from other reviewers before provide the best solution. There is no doubt that your comment is valid.
> 
> Thanks,
> drew

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