linux-kernel - Re: [PATCH v4 2/5] x86/kexec: do unconditional WBINVD for bare-metal in relocate

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Date: Thu, 18 Apr 2024 08:49:49 -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 2/5] x86/kexec: do unconditional WBINVD for bare-metal
 in relocate_kernel()

On 4/18/24 06:48, Kai Huang wrote:
> Both SME and TDX can leave caches in incoherent state due to memory
> encryption.  During kexec, the caches must be flushed before jumping to
> the second kernel to avoid silent memory corruption to the second kernel.
> 
> During kexec, the WBINVD in stop_this_cpu() flushes caches for all
> remote cpus when they are being stopped.  For SME, the WBINVD in
> relocate_kernel() flushes the cache for the last running cpu (which is
> executing the kexec).
> 
> Similarly, to support kexec for TDX host, after stopping all remote cpus
> with cache flushed, the kernel needs to flush cache for the last running
> cpu.
> 
> Use the existing WBINVD in relocate_kernel() to cover TDX host as well.
> 
> However, instead of sprinkling around vendor-specific checks, just do
> unconditional WBINVD to cover both SME and TDX.  Kexec is not a fast path
> so having one additional WBINVD for platforms w/o SME/TDX is acceptable.
> 
> But only do WBINVD for bare-metal because TDX guests and SEV-ES/SEV-SNP
> guests will get unexpected (and yet unnecessary) exception (#VE or #VC)
> which the kernel is unable to handle at this stage.
> 
> Signed-off-by: Kai Huang <kai.huang@...el.com>
> Reviewed-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> Cc: Tom Lendacky <thomas.lendacky@....com>
> Cc: Dave Young <dyoung@...hat.com>

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

> ---