linux-kernel - Re: [PATCH v6 04/15] x86/pvh: Use fixed_percpu

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Message-ID: <CAGudoHEQbBEbjXKfPR6+ktxj5GzBpQ+CjMXi+G7wqR-UQmW9Fw@mail.gmail.com>
Date: Wed, 5 Mar 2025 00:49:36 +0100
From: Mateusz Guzik <mjguzik@...il.com>
To: "H. Peter Anvin" <hpa@...or.com>
Cc: Brian Gerst <brgerst@...il.com>, linux-kernel@...r.kernel.org, x86@...nel.org, 
	Ingo Molnar <mingo@...nel.org>, Thomas Gleixner <tglx@...utronix.de>, Borislav Petkov <bp@...en8.de>, 
	Ard Biesheuvel <ardb@...nel.org>, Uros Bizjak <ubizjak@...il.com>
Subject: Re: [PATCH v6 04/15] x86/pvh: Use fixed_percpu_data for early boot GSBASE

On Wed, Mar 5, 2025 at 12:45 AM H. Peter Anvin <hpa@...or.com> wrote:
>
> On March 4, 2025 2:26:20 PM PST, Mateusz Guzik <mjguzik@...il.com> 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@...il.com>
> >> Reviewed-by: Ard Biesheuvel <ardb@...nel.org>
> >> ---
> >>  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.
-- 
Mateusz Guzik <mjguzik gmail.com>