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

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Message-ID: <uvm2vudogp3mkkhpx723l6nmmy3dzivneoyaacx6cfbbrlvjqc@wc5q7m27mmzr>
Date: Wed, 31 Jan 2024 14:58:57 +0200
From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To: Dave Hansen <dave.hansen@...ux.intel.com>, 
	Andy Lutomirski <luto@...nel.org>, Peter Zijlstra <peterz@...radead.org>
Cc: 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 24, 2024 at 03:06:50PM +0200, Kirill A. Shutemov wrote:
> The programming protocol for the PAT MSR follows the MTRR programming
> protocol. However, this protocol is cumbersome and requires disabling
> caching (CR0.CD=1), which is not possible on some platforms.
> 
> Specifically, a TDX guest is not allowed to set CR0.CD. It triggers
> a #VE exception.
> 
> Turned out the requirement to follow the MTRR programming protocol for
> PAT programming is unnecessarily strict. The new Intel Software
> Developer Manual[1] (December 2023) relaxes this requirement. Please
> refer to the section titled "Programming the PAT" for more information.
> 
> The AMD documentation does not link PAT programming to MTRR.
> 
> The kernel only needs to flush the TLB after updating the PAT MSR. The
> set_memory code already takes care of flushing the TLB and cache when
> changing the memory type of a page.
> 
> [1] http://www.intel.com/sdm
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> Reviewed-by: Juergen Gross <jgross@...e.com>

Any feedback?

-- 
  Kiryl Shutsemau / Kirill A. Shutemov