linux-kernel - Re: [PATCH 1/4] x86/coco: Add a new CC attribute to unify cache flush during kexec

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Date: Mon, 19 Feb 2024 21:32:22 +0100
From: Borislav Petkov <bp@...en8.de>
To: Tom Lendacky <thomas.lendacky@....com>
Cc: "Huang, Kai" <kai.huang@...el.com>, linux-kernel@...r.kernel.org,
	x86@...nel.org, dave.hansen@...el.com,
	kirill.shutemov@...ux.intel.com, tglx@...utronix.de,
	mingo@...hat.com, hpa@...or.com, luto@...nel.org,
	peterz@...radead.org, chao.gao@...el.com, bhe@...hat.com,
	nik.borisov@...e.com, pbonzini@...hat.com
Subject: Re: [PATCH 1/4] x86/coco: Add a new CC attribute to unify cache
 flush during kexec

On Mon, Feb 19, 2024 at 01:45:37PM -0600, Tom Lendacky wrote:
> This change won't return the correct answer. The check needs to remain
> against the sev_status value.

Feel free to explain because this patch is confusing me.

> > So you can't put it before the if - just slap it in both branches. Geez!
> 
> I think that will still work because sme_me_mask and sev_status will both be
> 0 on bare-metal if 'msr & MSR_AMD64_SYSCFG_MEM_ENCRYPT' doesn't evaluate to
> true. However, that will cause any platform that hasn't enabled memory
> encryption (see SYS_CFG MSR), to also perform the WBINVD.

If it keeps the code simpler I don't mind. That's so not a fast path.

> That won't work, because the current system may not have SME active. The
> cases that needs to be caught are kexec'ing from a mem_encrypt=off to a
> mem_encrypt=on or from a mem_encrypt=on to a mem_encrypt=off.

And I'm saying, we should keep it simple and simply WBINVD on SME
capable machines, regardless of the encryption setting.

Any downsides to that which are actually real and noticeable?

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette