Re: [PATCH 1/4] x86/percpu: Use explicit segment registers in lib/cmpxchg{8,16}b_emu.S

[Uros Bizjak]

On Thu, Oct 12, 2023 at 7:45 PM Brian Gerst <brgerst@xxxxxxxxx> wrote:
>
> On Thu, Oct 12, 2023 at 12:13 PM Uros Bizjak <ubizjak@xxxxxxxxx> wrote:
> >
> > PER_CPU_VAR macro is intended to be applied to a symbol, it is not
> > intended to be used as a selector between %fs and %gs segment
> > registers for general operands.
> >
> > The address to these emulation functions is passed in a register, so
> > use explicit segment registers to access percpu variable instead.
> >
> > Also add a missing function comment to this_cpu_cmpxchg8b_emu.
> >
> > No functional changes intended.
> >
> > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> > Cc: Borislav Petkov <bp@xxxxxxxxx>
> > Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> > Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > Signed-off-by: Uros Bizjak <ubizjak@xxxxxxxxx>
> > ---
> >  arch/x86/lib/cmpxchg16b_emu.S | 12 ++++++------
> >  arch/x86/lib/cmpxchg8b_emu.S  | 30 +++++++++++++++++++++---------
> >  2 files changed, 27 insertions(+), 15 deletions(-)
> >
> > diff --git a/arch/x86/lib/cmpxchg16b_emu.S b/arch/x86/lib/cmpxchg16b_emu.S
> > index 6962df315793..2bd8b89bce75 100644
> > --- a/arch/x86/lib/cmpxchg16b_emu.S
> > +++ b/arch/x86/lib/cmpxchg16b_emu.S
> > @@ -23,14 +23,14 @@ SYM_FUNC_START(this_cpu_cmpxchg16b_emu)
> >         cli
> >
> >         /* if (*ptr == old) */
> > -       cmpq    PER_CPU_VAR(0(%rsi)), %rax
> > +       cmpq    %gs:(%rsi), %rax
> >         jne     .Lnot_same
> > -       cmpq    PER_CPU_VAR(8(%rsi)), %rdx
> > +       cmpq    %gs:8(%rsi), %rdx
> >         jne     .Lnot_same
> >
> >         /* *ptr = new */
> > -       movq    %rbx, PER_CPU_VAR(0(%rsi))
> > -       movq    %rcx, PER_CPU_VAR(8(%rsi))
> > +       movq    %rbx, %gs:(%rsi)
> > +       movq    %rcx, %gs:8(%rsi)
> >
> >         /* set ZF in EFLAGS to indicate success */
> >         orl     $X86_EFLAGS_ZF, (%rsp)
> > @@ -42,8 +42,8 @@ SYM_FUNC_START(this_cpu_cmpxchg16b_emu)
> >         /* *ptr != old */
> >
> >         /* old = *ptr */
> > -       movq    PER_CPU_VAR(0(%rsi)), %rax
> > -       movq    PER_CPU_VAR(8(%rsi)), %rdx
> > +       movq    %gs:(%rsi), %rax
> > +       movq    %gs:8(%rsi), %rdx
> >
> >         /* clear ZF in EFLAGS to indicate failure */
> >         andl    $(~X86_EFLAGS_ZF), (%rsp)
> > diff --git a/arch/x86/lib/cmpxchg8b_emu.S b/arch/x86/lib/cmpxchg8b_emu.S
> > index 49805257b125..b7d68d5e2d31 100644
> > --- a/arch/x86/lib/cmpxchg8b_emu.S
> > +++ b/arch/x86/lib/cmpxchg8b_emu.S
> > @@ -24,12 +24,12 @@ SYM_FUNC_START(cmpxchg8b_emu)
> >         pushfl
> >         cli
> >
> > -       cmpl    0(%esi), %eax
> > +       cmpl    (%esi), %eax
> >         jne     .Lnot_same
> >         cmpl    4(%esi), %edx
> >         jne     .Lnot_same
> >
> > -       movl    %ebx, 0(%esi)
> > +       movl    %ebx, (%esi)
> >         movl    %ecx, 4(%esi)
> >
> >         orl     $X86_EFLAGS_ZF, (%esp)
> > @@ -38,7 +38,7 @@ SYM_FUNC_START(cmpxchg8b_emu)
> >         RET
> >
> >  .Lnot_same:
> > -       movl    0(%esi), %eax
> > +       movl    (%esi), %eax
> >         movl    4(%esi), %edx
> >
> >         andl    $(~X86_EFLAGS_ZF), (%esp)
> > @@ -53,18 +53,30 @@ EXPORT_SYMBOL(cmpxchg8b_emu)
> >
> >  #ifndef CONFIG_UML
> >
> > +/*
> > + * Emulate 'cmpxchg8b %fs:(%esi)'
> > + *
> > + * Inputs:
> > + * %esi : memory location to compare
> > + * %eax : low 32 bits of old value
> > + * %edx : high 32 bits of old value
> > + * %ebx : low 32 bits of new value
> > + * %ecx : high 32 bits of new value
> > + *
> > + * Notably this is not LOCK prefixed and is not safe against NMIs
> > + */
> >  SYM_FUNC_START(this_cpu_cmpxchg8b_emu)
> >
> >         pushfl
> >         cli
> >
> > -       cmpl    PER_CPU_VAR(0(%esi)), %eax
> > +       cmpl    %fs:(%esi), %eax
> >         jne     .Lnot_same2
> > -       cmpl    PER_CPU_VAR(4(%esi)), %edx
> > +       cmpl    %fs:4(%esi), %edx
> >         jne     .Lnot_same2
> >
> > -       movl    %ebx, PER_CPU_VAR(0(%esi))
> > -       movl    %ecx, PER_CPU_VAR(4(%esi))
> > +       movl    %ebx, %fs:(%esi)
> > +       movl    %ecx, %fs:4(%esi)
> >
> >         orl     $X86_EFLAGS_ZF, (%esp)
> >
> > @@ -72,8 +84,8 @@ SYM_FUNC_START(this_cpu_cmpxchg8b_emu)
> >         RET
> >
> >  .Lnot_same2:
> > -       movl    PER_CPU_VAR(0(%esi)), %eax
> > -       movl    PER_CPU_VAR(4(%esi)), %edx
> > +       movl    %fs:(%esi), %eax
> > +       movl    %fs:4(%esi), %edx
> >
> >         andl    $(~X86_EFLAGS_ZF), (%esp)
> >
> > --
> > 2.41.0
> >
>
> This will break on !SMP builds, where per-cpu variables are just
> regular data and not accessed with a segment prefix.

Ugh, indeed. Let me rethink this a bit.

Thanks,
Uros.