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Message-ID: <CAAhV-H6KtgTZJJc55ZB00p4oxrhOUvf9qn4Ls1mzDoO9QQ9gLw@mail.gmail.com>
Date: Tue, 23 Dec 2025 10:41:17 +0800
From: Huacai Chen <chenhuacai@...nel.org>
To: lixianglai <lixianglai@...ngson.cn>
Cc: Bibo Mao <maobibo@...ngson.cn>, loongarch@...ts.linux.dev,
linux-kernel@...r.kernel.org, kvm@...r.kernel.org, stable@...r.kernel.org,
WANG Xuerui <kernel@...0n.name>, Tianrui Zhao <zhaotianrui@...ngson.cn>,
Charlie Jenkins <charlie@...osinc.com>, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH V2 1/2] LoongArch: KVM: Compile the switch.S file directly
into the kernel
Hi, Xianglai,
On Tue, Dec 23, 2025 at 10:38 AM lixianglai <lixianglai@...ngson.cn> wrote:
>
> Hi Bibo Mao:
> >
> >
> > On 2025/12/22 下午7:34, Xianglai Li wrote:
> >> If we directly compile the switch.S file into the kernel, the address of
> >> the kvm_exc_entry function will definitely be within the DMW memory
> >> area.
> >> Therefore, we will no longer need to perform a copy relocation of
> >> kvm_exc_entry.
> >>
> >> Based on the above description, compile switch.S directly into the
> >> kernel,
> >> and then remove the copy relocation execution logic for the
> >> kvm_exc_entry
> >> function.
> >>
> >> Cc: stable@...r.kernel.org
> >> Signed-off-by: Xianglai Li <lixianglai@...ngson.cn>
> >> ---
> >> Cc: Huacai Chen <chenhuacai@...nel.org>
> >> Cc: WANG Xuerui <kernel@...0n.name>
> >> Cc: Tianrui Zhao <zhaotianrui@...ngson.cn>
> >> Cc: Bibo Mao <maobibo@...ngson.cn>
> >> Cc: Charlie Jenkins <charlie@...osinc.com>
> >> Cc: Xianglai Li <lixianglai@...ngson.cn>
> >> Cc: Thomas Gleixner <tglx@...utronix.de>
> >>
> >> arch/loongarch/Kbuild | 2 +-
> >> arch/loongarch/include/asm/asm-prototypes.h | 21 +++++++++++++
> >> arch/loongarch/include/asm/kvm_host.h | 3 --
> >> arch/loongarch/kvm/Makefile | 2 +-
> >> arch/loongarch/kvm/main.c | 35 ++-------------------
> >> arch/loongarch/kvm/switch.S | 22 ++++++++++---
> >> 6 files changed, 43 insertions(+), 42 deletions(-)
> >>
> >> diff --git a/arch/loongarch/Kbuild b/arch/loongarch/Kbuild
> >> index beb8499dd8ed..1c7a0dbe5e72 100644
> >> --- a/arch/loongarch/Kbuild
> >> +++ b/arch/loongarch/Kbuild
> >> @@ -3,7 +3,7 @@ obj-y += mm/
> >> obj-y += net/
> >> obj-y += vdso/
> >> -obj-$(CONFIG_KVM) += kvm/
> >> +obj-$(subst m,y,$(CONFIG_KVM)) += kvm/
> >> # for cleaning
> >> subdir- += boot
> >> diff --git a/arch/loongarch/include/asm/asm-prototypes.h
> >> b/arch/loongarch/include/asm/asm-prototypes.h
> >> index 704066b4f736..e8ce153691e5 100644
> >> --- a/arch/loongarch/include/asm/asm-prototypes.h
> >> +++ b/arch/loongarch/include/asm/asm-prototypes.h
> >> @@ -20,3 +20,24 @@ asmlinkage void noinstr __no_stack_protector
> >> ret_from_kernel_thread(struct task_
> >> struct pt_regs *regs,
> >> int (*fn)(void *),
> >> void *fn_arg);
> >> +
> >> +struct kvm_run;
> >> +struct kvm_vcpu;
> >> +
> >> +void kvm_exc_entry(void);
> >> +int kvm_enter_guest(struct kvm_run *run, struct kvm_vcpu *vcpu);
> >> +
> >> +struct loongarch_fpu;
> >> +
> >> +#ifdef CONFIG_CPU_HAS_LSX
> >> +void kvm_save_lsx(struct loongarch_fpu *fpu);
> >> +void kvm_restore_lsx(struct loongarch_fpu *fpu);
> >> +#endif
> >> +
> >> +#ifdef CONFIG_CPU_HAS_LASX
> >> +void kvm_save_lasx(struct loongarch_fpu *fpu);
> >> +void kvm_restore_lasx(struct loongarch_fpu *fpu);
> >> +#endif
> >> +
> >> +void kvm_save_fpu(struct loongarch_fpu *fpu);
> >> +void kvm_restore_fpu(struct loongarch_fpu *fpu);
> >> diff --git a/arch/loongarch/include/asm/kvm_host.h
> >> b/arch/loongarch/include/asm/kvm_host.h
> >> index e4fe5b8e8149..1a1be10e3803 100644
> >> --- a/arch/loongarch/include/asm/kvm_host.h
> >> +++ b/arch/loongarch/include/asm/kvm_host.h
> >> @@ -85,7 +85,6 @@ struct kvm_context {
> >> struct kvm_world_switch {
> >> int (*exc_entry)(void);
> >> int (*enter_guest)(struct kvm_run *run, struct kvm_vcpu *vcpu);
> >> - unsigned long page_order;
> >> };
> >> #define MAX_PGTABLE_LEVELS 4
> >> @@ -347,8 +346,6 @@ void kvm_exc_entry(void);
> >> int kvm_enter_guest(struct kvm_run *run, struct kvm_vcpu *vcpu);
> >> extern unsigned long vpid_mask;
> >> -extern const unsigned long kvm_exception_size;
> >> -extern const unsigned long kvm_enter_guest_size;
> >> extern struct kvm_world_switch *kvm_loongarch_ops;
> >> #define SW_GCSR (1 << 0)
> >> diff --git a/arch/loongarch/kvm/Makefile b/arch/loongarch/kvm/Makefile
> >> index cb41d9265662..fe665054f824 100644
> >> --- a/arch/loongarch/kvm/Makefile
> >> +++ b/arch/loongarch/kvm/Makefile
> >> @@ -11,7 +11,7 @@ kvm-y += exit.o
> >> kvm-y += interrupt.o
> >> kvm-y += main.o
> >> kvm-y += mmu.o
> >> -kvm-y += switch.o
> >> +obj-y += switch.o
> >> kvm-y += timer.o
> >> kvm-y += tlb.o
> >> kvm-y += vcpu.o
> >> diff --git a/arch/loongarch/kvm/main.c b/arch/loongarch/kvm/main.c
> >> index 80ea63d465b8..67d234540ed4 100644
> >> --- a/arch/loongarch/kvm/main.c
> >> +++ b/arch/loongarch/kvm/main.c
> >> @@ -340,8 +340,7 @@ void kvm_arch_disable_virtualization_cpu(void)
> >> static int kvm_loongarch_env_init(void)
> >> {
> >> - int cpu, order, ret;
> >> - void *addr;
> >> + int cpu, ret;
> >> struct kvm_context *context;
> >> vmcs = alloc_percpu(struct kvm_context);
> >> @@ -357,30 +356,8 @@ static int kvm_loongarch_env_init(void)
> >> return -ENOMEM;
> >> }
> >> - /*
> >> - * PGD register is shared between root kernel and kvm hypervisor.
> >> - * So world switch entry should be in DMW area rather than TLB area
> >> - * to avoid page fault reenter.
> >> - *
> >> - * In future if hardware pagetable walking is supported, we won't
> >> - * need to copy world switch code to DMW area.
> >> - */
> >> - order = get_order(kvm_exception_size + kvm_enter_guest_size);
> >> - addr = (void *)__get_free_pages(GFP_KERNEL, order);
> >> - if (!addr) {
> >> - free_percpu(vmcs);
> >> - vmcs = NULL;
> >> - kfree(kvm_loongarch_ops);
> >> - kvm_loongarch_ops = NULL;
> >> - return -ENOMEM;
> >> - }
> >> -
> >> - memcpy(addr, kvm_exc_entry, kvm_exception_size);
> >> - memcpy(addr + kvm_exception_size, kvm_enter_guest,
> >> kvm_enter_guest_size);
> >> - flush_icache_range((unsigned long)addr, (unsigned long)addr +
> >> kvm_exception_size + kvm_enter_guest_size);
> >> - kvm_loongarch_ops->exc_entry = addr;
> >> - kvm_loongarch_ops->enter_guest = addr + kvm_exception_size;
> >> - kvm_loongarch_ops->page_order = order;
> >> + kvm_loongarch_ops->exc_entry = (void *)kvm_exc_entry;
> >> + kvm_loongarch_ops->enter_guest = (void *)kvm_enter_guest;
> >> vpid_mask = read_csr_gstat();
> >> vpid_mask = (vpid_mask & CSR_GSTAT_GIDBIT) >>
> >> CSR_GSTAT_GIDBIT_SHIFT;
> >> @@ -414,16 +391,10 @@ static int kvm_loongarch_env_init(void)
> >> static void kvm_loongarch_env_exit(void)
> >> {
> >> - unsigned long addr;
> >> -
> >> if (vmcs)
> >> free_percpu(vmcs);
> >> if (kvm_loongarch_ops) {
> >> - if (kvm_loongarch_ops->exc_entry) {
> >> - addr = (unsigned long)kvm_loongarch_ops->exc_entry;
> >> - free_pages(addr, kvm_loongarch_ops->page_order);
> >> - }
> >> kfree(kvm_loongarch_ops);
> >> }
> >> diff --git a/arch/loongarch/kvm/switch.S b/arch/loongarch/kvm/switch.S
> >> index f1768b7a6194..93845ce53651 100644
> >> --- a/arch/loongarch/kvm/switch.S
> >> +++ b/arch/loongarch/kvm/switch.S
> >> @@ -5,6 +5,7 @@
> >> #include <linux/linkage.h>
> >> #include <asm/asm.h>
> >> +#include <asm/page.h>
> >> #include <asm/asmmacro.h>
> >> #include <asm/loongarch.h>
> >> #include <asm/regdef.h>
> >> @@ -100,10 +101,18 @@
> >> * - is still in guest mode, such as pgd table/vmid
> >> registers etc,
> >> * - will fix with hw page walk enabled in future
> >> * load kvm_vcpu from reserved CSR KVM_VCPU_KS, and save a2 to
> >> KVM_TEMP_KS
> >> + *
> >> + * PGD register is shared between root kernel and kvm hypervisor.
> >> + * So world switch entry should be in DMW area rather than TLB area
> >> + * to avoid page fault reenter.
> >> + *
> >> + * In future if hardware pagetable walking is supported, we won't
> >> + * need to copy world switch code to DMW area.
> >> */
> >> .text
> >> .cfi_sections .debug_frame
> >> SYM_CODE_START(kvm_exc_entry)
> >> + .p2align PAGE_SHIFT
I'm not sure, but if this line can be moved after .text, it seems better.
Huacai
> >> UNWIND_HINT_UNDEFINED
> >> csrwr a2, KVM_TEMP_KS
> >> csrrd a2, KVM_VCPU_KS
> >> @@ -190,8 +199,8 @@ ret_to_host:
> >> kvm_restore_host_gpr a2
> >> jr ra
> >> -SYM_INNER_LABEL(kvm_exc_entry_end, SYM_L_LOCAL)
> >> SYM_CODE_END(kvm_exc_entry)
> >> +EXPORT_SYMBOL(kvm_exc_entry)
> >> /*
> >> * int kvm_enter_guest(struct kvm_run *run, struct kvm_vcpu *vcpu)
> >> @@ -215,8 +224,8 @@ SYM_FUNC_START(kvm_enter_guest)
> >> /* Save kvm_vcpu to kscratch */
> >> csrwr a1, KVM_VCPU_KS
> >> kvm_switch_to_guest
> >> -SYM_INNER_LABEL(kvm_enter_guest_end, SYM_L_LOCAL)
> >> SYM_FUNC_END(kvm_enter_guest)
> >> +EXPORT_SYMBOL(kvm_enter_guest)
> >> SYM_FUNC_START(kvm_save_fpu)
> >> fpu_save_csr a0 t1
> >> @@ -224,6 +233,7 @@ SYM_FUNC_START(kvm_save_fpu)
> >> fpu_save_cc a0 t1 t2
> >> jr ra
> >> SYM_FUNC_END(kvm_save_fpu)
> >> +EXPORT_SYMBOL(kvm_save_fpu)
> > one small nit, could EXPORT_SYMBOL_FOR_KVM() be used here compared
> > with EXPORT_SYMBOL()?
> >
>
> Ok! will fix it in next version
> Thanks!
> Xianglai.
>
> > Regards
> > Bibo Mao
> >> SYM_FUNC_START(kvm_restore_fpu)
> >> fpu_restore_double a0 t1
> >> @@ -231,6 +241,7 @@ SYM_FUNC_START(kvm_restore_fpu)
> >> fpu_restore_cc a0 t1 t2
> >> jr ra
> >> SYM_FUNC_END(kvm_restore_fpu)
> >> +EXPORT_SYMBOL(kvm_restore_fpu)
> >> #ifdef CONFIG_CPU_HAS_LSX
> >> SYM_FUNC_START(kvm_save_lsx)
> >> @@ -239,6 +250,7 @@ SYM_FUNC_START(kvm_save_lsx)
> >> lsx_save_data a0 t1
> >> jr ra
> >> SYM_FUNC_END(kvm_save_lsx)
> >> +EXPORT_SYMBOL(kvm_save_lsx)
> >> SYM_FUNC_START(kvm_restore_lsx)
> >> lsx_restore_data a0 t1
> >> @@ -246,6 +258,7 @@ SYM_FUNC_START(kvm_restore_lsx)
> >> fpu_restore_csr a0 t1 t2
> >> jr ra
> >> SYM_FUNC_END(kvm_restore_lsx)
> >> +EXPORT_SYMBOL(kvm_restore_lsx)
> >> #endif
> >> #ifdef CONFIG_CPU_HAS_LASX
> >> @@ -255,6 +268,7 @@ SYM_FUNC_START(kvm_save_lasx)
> >> lasx_save_data a0 t1
> >> jr ra
> >> SYM_FUNC_END(kvm_save_lasx)
> >> +EXPORT_SYMBOL(kvm_save_lasx)
> >> SYM_FUNC_START(kvm_restore_lasx)
> >> lasx_restore_data a0 t1
> >> @@ -262,10 +276,8 @@ SYM_FUNC_START(kvm_restore_lasx)
> >> fpu_restore_csr a0 t1 t2
> >> jr ra
> >> SYM_FUNC_END(kvm_restore_lasx)
> >> +EXPORT_SYMBOL(kvm_restore_lasx)
> >> #endif
> >> - .section ".rodata"
> >> -SYM_DATA(kvm_exception_size, .quad kvm_exc_entry_end - kvm_exc_entry)
> >> -SYM_DATA(kvm_enter_guest_size, .quad kvm_enter_guest_end -
> >> kvm_enter_guest)
> >> #ifdef CONFIG_CPU_HAS_LBT
> >> STACK_FRAME_NON_STANDARD kvm_restore_fpu
> >>
>
>
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