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

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Message-ID: <56A91DC4-1A8C-4134-976E-BBCBF9BC784F@zytor.com>
Date: Tue, 04 Mar 2025 15:44:47 -0800
From: "H. Peter Anvin" <hpa@...or.com>
To: Mateusz Guzik <mjguzik@...il.com>, Brian Gerst <brgerst@...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>, 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 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?