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Date: Tue, 24 Oct 2023 10:11:57 -0400
From: Brian Gerst <brgerst@...il.com>
To: Uros Bizjak <ubizjak@...il.com>
Cc: linux-kernel@...r.kernel.org, x86@...nel.org,
Ingo Molnar <mingo@...nel.org>,
Thomas Gleixner <tglx@...utronix.de>,
Borislav Petkov <bp@...en8.de>,
"H . Peter Anvin" <hpa@...or.com>,
Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 4/9] x86/percpu/64: Remove fixed_percpu_data
On Tue, Oct 24, 2023 at 8:33 AM Uros Bizjak <ubizjak@...il.com> wrote:
>
> On Mon, Oct 23, 2023 at 11:17 PM Brian Gerst <brgerst@...il.com> wrote:
> >
> > Now that the stack protector canary value is a normal percpu variable,
> > fixed_percpu_data is unused and can be removed.
> >
> > Signed-off-by: Brian Gerst <brgerst@...il.com>
> > ---
> > arch/x86/include/asm/processor.h | 13 +++++--------
> > arch/x86/kernel/cpu/common.c | 4 ----
> > arch/x86/kernel/head_64.S | 12 ++++++------
> > arch/x86/kernel/vmlinux.lds.S | 6 ------
> > arch/x86/tools/relocs.c | 1 -
> > arch/x86/xen/xen-head.S | 12 ++++++++----
> > 6 files changed, 19 insertions(+), 29 deletions(-)
> >
> > diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
> > index 04371f60e3c6..48c31b8e3e72 100644
> > --- a/arch/x86/include/asm/processor.h
> > +++ b/arch/x86/include/asm/processor.h
> > @@ -393,16 +393,13 @@ struct irq_stack {
> > } __aligned(IRQ_STACK_SIZE);
> >
> > #ifdef CONFIG_X86_64
> > -struct fixed_percpu_data {
> > - char gs_base[40];
> > -};
> > -
> > -DECLARE_PER_CPU_FIRST(struct fixed_percpu_data, fixed_percpu_data) __visible;
> > -DECLARE_INIT_PER_CPU(fixed_percpu_data);
> > -
> > static inline unsigned long cpu_kernelmode_gs_base(int cpu)
> > {
> > - return (unsigned long)per_cpu(fixed_percpu_data.gs_base, cpu);
> > +#ifdef CONFIG_SMP
> > + return per_cpu_offset(cpu);
> > +#else
> > + return 0;
> > +#endif
> > }
> >
> > extern asmlinkage void entry_SYSCALL32_ignore(void);
> > diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> > index f9c8bd27b642..a44fd3ad460e 100644
> > --- a/arch/x86/kernel/cpu/common.c
> > +++ b/arch/x86/kernel/cpu/common.c
> > @@ -2051,10 +2051,6 @@ DEFINE_PER_CPU_ALIGNED(struct pcpu_hot, pcpu_hot) = {
> > EXPORT_PER_CPU_SYMBOL(pcpu_hot);
> >
> > #ifdef CONFIG_X86_64
> > -DEFINE_PER_CPU_FIRST(struct fixed_percpu_data,
> > - fixed_percpu_data) __aligned(PAGE_SIZE) __visible;
> > -EXPORT_PER_CPU_SYMBOL_GPL(fixed_percpu_data);
> > -
> > static void wrmsrl_cstar(unsigned long val)
> > {
> > /*
> > diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
> > index 3dcabbc49149..f2453eb38417 100644
> > --- a/arch/x86/kernel/head_64.S
> > +++ b/arch/x86/kernel/head_64.S
> > @@ -72,9 +72,14 @@ SYM_CODE_START_NOALIGN(startup_64)
> >
> > /* Setup GSBASE to allow stack canary access for C code */
> > movl $MSR_GS_BASE, %ecx
> > - leaq INIT_PER_CPU_VAR(fixed_percpu_data)(%rip), %rdx
> > +#ifdef CONFIG_SMP
> > + leaq __per_cpu_load(%rip), %rdx
> > movl %edx, %eax
> > shrq $32, %rdx
> > +#else
> > + xorl %eax, %eax
> > + xorl %edx, %edx
> > +#endif
> > wrmsr
> >
> > call startup_64_setup_env
> > @@ -345,15 +350,10 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
> >
> > /* Set up %gs.
> > *
> > - * The base of %gs always points to fixed_percpu_data. If the
> > - * stack protector canary is enabled, it is located at %gs:40.
> > * Note that, on SMP, the boot cpu uses init data section until
> > * the per cpu areas are set up.
> > */
> > movl $MSR_GS_BASE,%ecx
> > -#ifndef CONFIG_SMP
> > - leaq INIT_PER_CPU_VAR(fixed_percpu_data)(%rip), %rdx
> > -#endif
> > movl %edx, %eax
> > shrq $32, %rdx
> > wrmsr
> > diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
> > index 54a5596adaa6..c87dc8de2084 100644
> > --- a/arch/x86/kernel/vmlinux.lds.S
> > +++ b/arch/x86/kernel/vmlinux.lds.S
> > @@ -509,14 +509,8 @@ SECTIONS
> > */
> > #define INIT_PER_CPU(x) init_per_cpu__##x = ABSOLUTE(x) + __per_cpu_load
> > INIT_PER_CPU(gdt_page);
> > -INIT_PER_CPU(fixed_percpu_data);
> > INIT_PER_CPU(irq_stack_backing_store);
> >
> > -#ifdef CONFIG_SMP
> > -. = ASSERT((fixed_percpu_data == 0),
> > - "fixed_percpu_data is not at start of per-cpu area");
> > -#endif
> > -
> > #ifdef CONFIG_CPU_UNRET_ENTRY
> > . = ASSERT((retbleed_return_thunk & 0x3f) == 0, "retbleed_return_thunk not cacheline-aligned");
> > #endif
> > diff --git a/arch/x86/tools/relocs.c b/arch/x86/tools/relocs.c
> > index d30949e25ebd..3ccd9d4fcf9c 100644
> > --- a/arch/x86/tools/relocs.c
> > +++ b/arch/x86/tools/relocs.c
> > @@ -811,7 +811,6 @@ static void percpu_init(void)
> > * __per_cpu_load
> > *
> > * The "gold" linker incorrectly associates:
> > - * init_per_cpu__fixed_percpu_data
> > * init_per_cpu__gdt_page
> > */
> > static int is_percpu_sym(ElfW(Sym) *sym, const char *symname)
> > diff --git a/arch/x86/xen/xen-head.S b/arch/x86/xen/xen-head.S
> > index a0ea285878db..9ce0d9d268bb 100644
> > --- a/arch/x86/xen/xen-head.S
> > +++ b/arch/x86/xen/xen-head.S
> > @@ -53,14 +53,18 @@ SYM_CODE_START(startup_xen)
> >
> > /* Set up %gs.
> > *
> > - * The base of %gs always points to fixed_percpu_data. If the
> > - * stack protector canary is enabled, it is located at %gs:40.
> > * Note that, on SMP, the boot cpu uses init data section until
> > * the per cpu areas are set up.
> > */
> > movl $MSR_GS_BASE,%ecx
> > - movq $INIT_PER_CPU_VAR(fixed_percpu_data),%rax
> > - cdq
> > +#ifdef CONFIG_SMP
> > + leaq __per_cpu_load(%rip), %rdx
> > + movl %edx, %eax
> > + shrq $32, %rdx
> > +#else
> > + xorl %eax, %eax
> > + xorl %edx, %edx
> > +#endif
> > wrmsr
> >
> > mov %rsi, %rdi
>
> Please note there is another access to $MSR_GS_BASE in
> /arch/x86/platform/pvh/head.S around line 98. Should this be fixed,
> too?
Thanks for pointing that out. I missed it since it didn't reference
fixed_percpu_data. Fixed for v2.
Brian Gerst
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