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

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Message-ID: <11d96c35-9763-4e34-a3a2-3bc3384f88c9@suse.com>
Date: Tue, 2 Jan 2024 10:30:22 +0100
From: Juergen Gross <jgross@...e.com>
To: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
 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
Subject: Re: [PATCH] x86/pat: Simplifying the PAT programming protocol

On 28.12.23 13:58, 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>
> Cc: Juergen Gross <jgross@...e.com>

Assuming other vendors' cpus (e.g. Hygon) are fine with this:

Reviewed-by: Juergen Gross <jgross@...e.com>


Juergen


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