linux-kernel - Re: [PATCH, RESEND] x86/pat: Simplifying the PAT programming protocol

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Message-ID: <20240131202340.GLZbqsTJkeFQycXT0B@fat_crate.local>
Date: Wed, 31 Jan 2024 21:23:40 +0100
From: Borislav Petkov <bp@...en8.de>
To: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc: Dave Hansen <dave.hansen@...ux.intel.com>,
	Andy Lutomirski <luto@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Isaku Yamahata <isaku.yamahata@...el.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org, Juergen Gross <jgross@...e.com>
Subject: Re: [PATCH, RESEND] x86/pat: Simplifying the PAT programming protocol

On Wed, Jan 31, 2024 at 08:52:46PM +0200, Kirill A. Shutemov wrote:
> The second step is relevant for set_memory code that already does the
> flushing on changing memory type.

So the "relaxation" is the removal of that CR0.CD requirement?

> Our HW folks confirmed that the new optimized sequence works on all past
> processors that support PAT.

Your folks confirmed that for all released hw and for x86 hardware from
other vendors?

> Testing wouldn't hurt, but it cannot possibly prove that the new flow is
> safe by testing.

This basically says that I should not take this patch at all as there's
no way of even testing it?!

I hope I'm misunderstanding you.

> PAT MST was always virtualized, but we was not able to program it as we
> followed MTRR protocol that sets CR0.CD. And I covered it in the commit
> message:
> 
>     Specifically, a TDX guest is not allowed to set CR0.CD. It triggers
>     a #VE exception.

Ok, I'll extend that part once the rest has been sorted out.

Thx.

-- 
Regards/Gruss,
    Boris.

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