linux-kernel - Re: [PATCH v2 0/8] x86/mtrr: fix handling with PAT but without MTRR

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Date:   Wed, 15 Feb 2023 09:25:13 +0100
From:   Juergen Gross <jgross@...e.com>
To:     "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>
Cc:     "Ostrovsky, Boris" <boris.ostrovsky@...cle.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "Torvalds, Linus" <torvalds@...ux-foundation.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "kys@...rosoft.com" <kys@...rosoft.com>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "lists@...dbynature.de" <lists@...dbynature.de>,
        "hpa@...or.com" <hpa@...or.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "wei.liu@...nel.org" <wei.liu@...nel.org>,
        "Lutomirski, Andy" <luto@...nel.org>,
        "bp@...en8.de" <bp@...en8.de>, "Cui, Dexuan" <decui@...rosoft.com>,
        "mikelley@...rosoft.com" <mikelley@...rosoft.com>,
        "haiyangz@...rosoft.com" <haiyangz@...rosoft.com>,
        "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>
Subject: Re: [PATCH v2 0/8] x86/mtrr: fix handling with PAT but without MTRR

On 13.02.23 19:21, Edgecombe, Rick P wrote:
> On Mon, 2023-02-13 at 07:12 +0100, Juergen Gross wrote:
>>
>> Thanks for the report.
>>
>> I'll have a look. Probably I'll need to re-add the check for WB in
>> patch 7.
> 
> Sure, let me know if you need any more details about by setup.

I have reproduced the issue.

Adding back the test for WB will fix it, but I'm not sure this is really
what I should do.

The problem arises in case a large mapping is spanning multiple MTRRs,
even if they define the same caching type (uniform is set to 0 in this
case).

So the basic question for me is: shouldn't the semantics of uniform be
adpated? Today it means "the range is covered by only one MTRR or by
none". Looking at the use cases I'm wondering whether it shouldn't be
"the whole range has the same caching type".

Thoughts?


Juergen

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