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

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Message-ID: <tns2rrzk7vs3linnjevr24qyg4sm6hakndsgqvqsowqwwlrdcj@zus5wu6u3ju3>
Date: Tue, 4 Mar 2025 23:26:20 +0100
From: Mateusz Guzik <mjguzik@...il.com>
To: Brian Gerst <brgerst@...il.com>
Cc: linux-kernel@...r.kernel.org, x86@...nel.org, 
	Ingo Molnar <mingo@...nel.org>, "H . Peter Anvin" <hpa@...or.com>, 
	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 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.