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

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Date:   Mon, 13 Feb 2023 17:23:57 +0100
From:   Juergen Gross <jgross@...e.com>
To:     Christian Kujau <lists@...dbynature.de>, mikelley@...rosoft.com
Cc:     Linux regressions mailing list <regressions@...ts.linux.dev>,
        linux-kernel@...r.kernel.org, x86@...nel.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 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?

> Otherwise one would have to carry around that single revert manually until
> this patch series has landed in mainline, or convince all the
> distributions to do so :-\
> 
> Anyway, thanks for fixing this problem, I did not expect this to be such a
> complicated issue when I reported that thing :-)

Yes, I have opened a can of worms with my MTRR/PAT disentangling.


Juergen

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