linux-kernel - Re: [PATCH v4 1/5] x86/kexec: do unconditional WBINVD for bare-metal in stop_this

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Date: Thu, 18 Apr 2024 08:47:34 -0500
From: Tom Lendacky <thomas.lendacky@....com>
To: Kai Huang <kai.huang@...el.com>, linux-kernel@...r.kernel.org
Cc: x86@...nel.org, dave.hansen@...el.com, bp@...en8.de,
 kirill.shutemov@...ux.intel.com, tglx@...utronix.de, mingo@...hat.com,
 hpa@...or.com, luto@...nel.org, peterz@...radead.org,
 rick.p.edgecombe@...el.com, ashish.kalra@....com, chao.gao@...el.com,
 bhe@...hat.com, nik.borisov@...e.com, pbonzini@...hat.com, seanjc@...gle.com
Subject: Re: [PATCH v4 1/5] x86/kexec: do unconditional WBINVD for bare-metal
 in stop_this_cpu()

On 4/18/24 06:48, Kai Huang wrote:
> 

..

> Signed-off-by: Kai Huang <kai.huang@...el.com>
> Suggested-by: Borislav Petkov <bp@...en8.de>
> Cc: Tom Lendacky <thomas.lendacky@....com>
> Cc: Dave Young <dyoung@...hat.com>

Reviewed-by: Tom Lendacky <thomas.lendacky@....com>

> ---
> 
> v3 -> v4:
>   - Update part of changelog based on Kirill's version (with minor tweak).
>   - Use "exception (#VE or #VC)" for TDX and SEV-ES/SEV-SNP in changelog
>     and comments.  (Kirill, Tom)
>   - Point out "WBINVD is not necessary for TDX and SEV-ES/SEV-SNP guests"
>     in the comment.  (Tom)
> 
> v2 -> v3:
>   - Change to only do WBINVD for bare metal
> 
> ---
>   arch/x86/kernel/process.c | 19 +++++++++----------
>   1 file changed, 9 insertions(+), 10 deletions(-)
> 
> diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
> index b8441147eb5e..d3c904bfe874 100644
> --- a/arch/x86/kernel/process.c
> +++ b/arch/x86/kernel/process.c
> @@ -813,18 +813,17 @@ 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() isn't a fast path, just do WBINVD for bare-metal
> +	 * to cover both SME and TDX.  It isn't necessary to perform WBINVD
> +	 * in a guest and performing one could result in an exception (#VE
> +	 * or #VC) for a TDX or SEV-ES/SEV-SNP guest that the guest may
> +	 * not be able to handle (e.g., TDX guest panics if it sees #VE).
>   	 */
> -	if (c->extended_cpuid_level >= 0x8000001f && (cpuid_eax(0x8000001f) & BIT(0)))
> +	if (!boot_cpu_has(X86_FEATURE_HYPERVISOR))
>   		native_wbinvd();
>   
>   	/*