linux-kernel - Re: [PATCH v2 2/5] x86/kexec: do unconditional WBINVD in relocate

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Date: Wed, 20 Mar 2024 14:51:28 +0200
From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To: "Huang, Kai" <kai.huang@...el.com>
Cc: Tom Lendacky <thomas.lendacky@....com>, 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, ashish.kalra@....com, chao.gao@...el.com, bhe@...hat.com, 
	nik.borisov@...e.com, pbonzini@...hat.com, seanjc@...gle.com
Subject: Re: [PATCH v2 2/5] x86/kexec: do unconditional WBINVD in
 relocate_kernel()

On Wed, Mar 20, 2024 at 01:45:32PM +1300, Huang, Kai wrote:
> Anyway, regardless whether patch 1 will break TDX/SEV-ES/SEV-SNP guests, I
> think to resolve this, we can simply adjust our mindset from ...
> 
> 	"do unconditional WBINVD"
> 
> to ...
> 
> 	"do unconditional WBINVD when it can be done safely"
> 
> For now, AFAICT, only TDX guests and SEV-ES/SEV-SNP guests are such guests.
> 
> And they all report the CC_ATTR_GUEST_MEM_ENCRYPT flag as true, so we can
> change to only do WBINVD when the kernel sees that flag.
> 
> 	if (!cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT))
> 		native_wbinvd();
> 
> Alternatively, we can have a dedicated X86_FEATURE_NO_WBINVD and get it set
> for TDX/SEV-ES/SEV-SNP guests (and any guests if this is true), and do:
> 
> 	if (!boot_cpu_has(X86_FEATURE_NO_WBINVD))
> 		native_wbinvd();
> 
> It seems the first one is too generic (for any CoCo VMs), and the second one
> is better.
> 
> Any comments?

I like cc_platform_has() variant better. There's no good reason to invent
X86_FEATURE if we don't cases outside of CC.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov