Re: [PATCH v6 04/15] x86/pvh: Use fixed_percpu_data for early boot GSBASE

[Uros Bizjak]

On Wed, Mar 5, 2025 at 12:49 AM Mateusz Guzik <mjguzik@xxxxxxxxx> wrote:
>
> On Wed, Mar 5, 2025 at 12:45 AM H. Peter Anvin <hpa@xxxxxxxxx> wrote:
> >
> > On March 4, 2025 2:26:20 PM PST, Mateusz Guzik <mjguzik@xxxxxxxxx> wrote:
> > >On Thu, Jan 23, 2025 at 02:07:36PM -0500, Brian Gerst wrote:
> > >> Instead of having a private area for the stack canary, use
> > >> fixed_percpu_data for GSBASE like the native kernel.
> > >>
> > >> Signed-off-by: Brian Gerst <brgerst@xxxxxxxxx>
> > >> Reviewed-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
> > >> ---
> > >>  arch/x86/platform/pvh/head.S | 15 +++++++++------
> > >>  1 file changed, 9 insertions(+), 6 deletions(-)
> > >>
> > >> diff --git a/arch/x86/platform/pvh/head.S b/arch/x86/platform/pvh/head.S
> > >> index 4733a5f467b8..fa0072e0ca43 100644
> > >> --- a/arch/x86/platform/pvh/head.S
> > >> +++ b/arch/x86/platform/pvh/head.S
> > >> @@ -173,10 +173,15 @@ SYM_CODE_START(pvh_start_xen)
> > >>  1:
> > >>      UNWIND_HINT_END_OF_STACK
> > >>
> > >> -    /* Set base address in stack canary descriptor. */
> > >> -    mov $MSR_GS_BASE,%ecx
> > >> -    leal canary(%rip), %eax
> > >> -    xor %edx, %edx
> > >> +    /*
> > >> +     * Set up GSBASE.
> > >> +     * Note that, on SMP, the boot cpu uses init data section until
> > >> +     * the per cpu areas are set up.
> > >> +     */
> > >> +    movl $MSR_GS_BASE,%ecx
> > >> +    leaq INIT_PER_CPU_VAR(fixed_percpu_data)(%rip), %rdx
> > >> +    movq %edx, %eax
> > >
> > >       movl
> > >
> > >I'm bisecting perf breakage and landing on this commit breaks the build.
> >
> > Breaks the build how?
>
>   AS      arch/x86/platform/pvh/head.o
> arch/x86/platform/pvh/head.S: Assembler messages:
> arch/x86/platform/pvh/head.S:183: Error: incorrect register `%eax'
> used with `q' suffix
>
> Per the above, I presume it was meant to be a movl.

Please note that this part is further changed in [PATCH 8/15] [1], so
the breakage is temporary.

[1] https://lore.kernel.org/lkml/20250123190747.745588-9-brgerst@xxxxxxxxx/

Uros.