linux-kernel - Re: [PATCH v3 1/5] x86/kexec: do unconditional WBINVD for bare-metal in stop_this

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Date: Wed, 10 Apr 2024 17:12:49 +0300
From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To: Kai Huang <kai.huang@...el.com>
Cc: linux-kernel@...r.kernel.org, x86@...nel.org, dave.hansen@...el.com, 
	bp@...en8.de, tglx@...utronix.de, mingo@...hat.com, hpa@...or.com, 
	luto@...nel.org, peterz@...radead.org, rick.p.edgecombe@...el.com, 
	thomas.lendacky@....com, ashish.kalra@....com, chao.gao@...el.com, bhe@...hat.com, 
	nik.borisov@...e.com, pbonzini@...hat.com, seanjc@...gle.com
Subject: Re: [PATCH v3 1/5] x86/kexec: do unconditional WBINVD for bare-metal
 in stop_this_cpu()

On Mon, Apr 08, 2024 at 12:44:54AM +1200, Kai Huang wrote:
> TL;DR:

The commit message is waaay too verbose for no good reason. You don't
really need to repeat all the history around this code.

> ---
>  arch/x86/kernel/process.c | 18 ++++++++----------
>  1 file changed, 8 insertions(+), 10 deletions(-)
> 
> diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
> index b8441147eb5e..5ba8a9c1e47a 100644
> --- a/arch/x86/kernel/process.c
> +++ b/arch/x86/kernel/process.c
> @@ -813,18 +813,16 @@ void __noreturn stop_this_cpu(void *dummy)
>  	mcheck_cpu_clear(c);
>  
>  	/*
> -	 * Use wbinvd on processors that support SME. This provides support
> -	 * for performing a successful kexec when going from SME inactive
> -	 * to SME active (or vice-versa). The cache must be cleared so that
> -	 * if there are entries with the same physical address, both with and
> -	 * without the encryption bit, they don't race each other when flushed
> -	 * and potentially end up with the wrong entry being committed to
> -	 * memory.
> +	 * The kernel could leave caches in incoherent state on SME/TDX
> +	 * capable platforms.  Flush cache to avoid silent memory
> +	 * corruption for these platforms.
>  	 *
> -	 * Test the CPUID bit directly because the machine might've cleared
> -	 * X86_FEATURE_SME due to cmdline options.
> +	 * stop_this_cpu() is not a fast path, just do unconditional
> +	 * WBINVD for simplicity.  But only do WBINVD for bare-metal
> +	 * as TDX guests and SEV-ES/SEV-SNP guests will get unexpected
> +	 * (and unnecessary) #VE and may unable to handle.

s/#VE/exception/

On SEV it is #VC, not #VE.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov