linux-kernel - Re: [PATCH v3 1/6] x86/sme: Use percpu boolean to control wbinvd during kexec

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Message-ID: <20250701121218.GBaGPQold6Kw2M-nuc@fat_crate.local>
Date: Tue, 1 Jul 2025 14:12:18 +0200
From: Borislav Petkov <bp@...en8.de>
To: "Huang, Kai" <kai.huang@...el.com>
Cc: "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"ashish.kalra@....com" <ashish.kalra@....com>,
	"Hansen, Dave" <dave.hansen@...el.com>,
	"thomas.lendacky@....com" <thomas.lendacky@....com>,
	"kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
	"Chatre, Reinette" <reinette.chatre@...el.com>,
	"seanjc@...gle.com" <seanjc@...gle.com>,
	"pbonzini@...hat.com" <pbonzini@...hat.com>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"Yamahata, Isaku" <isaku.yamahata@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"nik.borisov@...e.com" <nik.borisov@...e.com>,
	"hpa@...or.com" <hpa@...or.com>,
	"peterz@...radead.org" <peterz@...radead.org>,
	"sagis@...gle.com" <sagis@...gle.com>,
	"Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
	"x86@...nel.org" <x86@...nel.org>,
	"Williams, Dan J" <dan.j.williams@...el.com>
Subject: Re: [PATCH v3 1/6] x86/sme: Use percpu boolean to control wbinvd
 during kexec

On Mon, Jun 30, 2025 at 11:34:34AM +0000, Huang, Kai wrote:
> Yeah I agree the text can be improved.  I tried to run AI to simplify but so
> far I am not quite happy about the result yet.  I'll try more.

Ask it to simplify it. I get it that you want to be exhaustive in your commit
message but there really is such thing as too much text.

Think of it this way: is the text I'm writing optimal when anyone needs to
read it in the future to know why a change has been done. If not, try to make
it so.

> Yeah I agree a single u32 + flags is better.  However this is the problem in
> the existing code (this patch only does renaming).
> 
> I think I can come up with a patch to clean this up and put it as the first
> patch of this series, or I can do a patch to clean this up after this series
> (either together with this series, or separately at a later time).  Which
> way do you prefer?

Clean ups go first, so yeah, please do a cleanup pre-patch.

>   /*
>    * The cache may be in an incoherent state (e.g., due to memory 
>    * encryption) and needs flushing during kexec.
>    */

Better than nothing. I'd try to explain with 1-2 sentences what can happen due
to memory encryption and why cache invalidation is required. So that the
comment is standalone and is not sending you on a wild goose chasing ride.

> IIUC the X86_FEATURE_SME could be cleared via 'clearcpuid' kernel cmdline.
> 
> Please also see my reply to Tom.

I know but we have never said that clearcpuid= should be used in production.
If you break the kernel using it, you get to keep the pieces. clearcpuid=
taints the kernel and screams bloody murder. So I'm not too worried about
that.

What is more relevant is this:

"I did verify that booting with mem_encrypt=off will start with
X86_FEATURE_SME set, the BSP will clear it and then all APs will not see
it set after that."

which should be put there in the comment.

Thx.

-- 
Regards/Gruss,
    Boris.

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