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

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Date:   Thu, 16 Feb 2023 06:35:31 +0100
From:   Juergen Gross <jgross@...e.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     "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>,
        "Ostrovsky, Boris" <boris.ostrovsky@...cle.com>,
        "peterz@...radead.org" <peterz@...radead.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 16.02.23 00:22, Linus Torvalds wrote:
> On Wed, Feb 15, 2023 at 12:25 AM Juergen Gross <jgross@...e.com> wrote:
>>
>> 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).
> 
> Oh, I think then you should fix uniform to be 1.
> 
> IOW, we should not think "multiple MTRRs" means "non-uniform". Only
> "different actual memory types" should mean non-uniformity.

Thanks for confirmation. I completely agree.

> If I remember correctly, there were good reasons to have overlapping
> MTRR's. In fact, you can generate a single MTRR that described a
> memory ttype that wasn't even contiguous if you had odd memory setups.
> 
> Intel definitely defines how overlapping MTRR's work, and "same types
> overlaps" is documented as a real thing.

Yes. And it is handled wrong in current code.

Handling it correctly will require quite some reworking of the code,
which I've already started to work on. I will defer the pud_set_huge()/
pmd_set_huge() modifying patch to after this rework.


Juergen

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