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

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Date: Thu, 1 Feb 2024 00:17:32 +0200
From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To: Borislav Petkov <bp@...en8.de>
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 09:23:40PM +0100, Borislav Petkov wrote:
> 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?

And double WBINVD if the machine has no X86_FEATURE_SELFSNOOP (before and
after TLB flush).

> > 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?

No. They can only talk for Intel CPUs. But AMD docs don't require MTTR
flow to begin with.

It is better to double-check on AMD side.

> > 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.

Testing can prove that the proposed patch is broken if a problem show ups.
But if you found no issue during testing you cannot say that the patch is
safe. You could just get lucky and don't hit pathological scenario with
broken cache/TLB aliases or something.

It is better to get confirmation from HW folks.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov