linux-kernel - Re: [PATCH v2 5/8] x86/mtrr: revert commit 90b926e68f50

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Date:   Mon, 13 Feb 2023 18:24:25 +0100
From:   Juergen Gross <jgross@...e.com>
To:     "Michael Kelley (LINUX)" <mikelley@...rosoft.com>,
        Christian Kujau <lists@...dbynature.de>
Cc:     Linux regressions mailing list <regressions@...ts.linux.dev>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v2 5/8] x86/mtrr: revert commit 90b926e68f50

On 13.02.23 18:01, Michael Kelley (LINUX) wrote:
> From: Juergen Gross <jgross@...e.com>
>>
>> On 13.02.23 12:46, Christian Kujau wrote:
>>> On Mon, 13 Feb 2023, Juergen Gross wrote:
>>>> On 10.02.23 19:59, Linux regression tracking (Thorsten Leemhuis) wrote:
>>>>> Hi, this is your Linux kernel regression tracker.
>>>>>
>>>>> On 09.02.23 08:22, Juergen Gross wrote:
>>>>>> Commit 90b926e68f50 ("x86/pat: Fix pat_x_mtrr_type() for MTRR disabled
>>>>>> case") has introduced a regression with Xen.
>>>>>>
>>>>>> Revert the patch.
>>>>>
>>>>> That regression you refer to is afaics one I'm tracking[1] that was
>>>>> introduced this cycle. That makes me wonder: could this patch be applied
>>>>> directly to fix the issue quickly? Or are patches 1 to 4 needed as well
>>>>> (or the whole series?) to avoid other problems?
>>>>
>>>> Patches 1-4 are needed, too, as otherwise the issue claimed to be fixed
>>>> with patch 5 would show up again.
>>>
>>> The (last?) -rc8 version was released yesterday. Would it be possible to
>>> include at least (only) the revert in mainline so that 6.2 will be
>>> released with a working storage configuration under Xen?
>>
>> Hmm, this would make Hyper-V SEV-SNP guests slow again.
>>
>> I'm not completely against it, but OTOH I'm a little bit biased as the
>> maintainer of the Xen code. :-)
>>
>> Michael, would you see major problems with doing the revert before having
>> the final patches for fixing your issue, too?
>>
> 
> I'm OK with doing the revert.  It's probably the right tradeoff for the
> broader community because the Hyper-V use case is more narrow and
> requires more curation for other reasons.  The use case is the Azure
> public cloud, and we can pretty much make sure that one of the solutions
> is applied to kernels used with SEV-SNP in that environment.

Thanks.

Boris, would you take the revert (patch 5 of my series) via x86/urgent, please?


Juergen

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