Re: [PATCH 0/3] x86: make pat and mtrr independent from each other

[Chuck Zmudzinski <brchuckz@xxxxxxxxxxxx>]

On 7/17/22 3:55 AM, Thorsten Leemhuis wrote:
> Hi Juergen!
>
> On 15.07.22 16:25, Juergen Gross wrote:
> > Today PAT can't be used without MTRR being available, unless MTRR is at
> > least configured via CONFIG_MTRR and the system is running as Xen PV
> > guest. In this case PAT is automatically available via the hypervisor,
> > but the PAT MSR can't be modified by the kernel and MTRR is disabled.
> > 
> > As an additional complexity the availability of PAT can't be queried
> > via pat_enabled() in the Xen PV case, as the lack of MTRR will set PAT
> > to be disabled. This leads to some drivers believing that not all cache
> > modes are available, resulting in failures or degraded functionality.
> > 
> > The same applies to a kernel built with no MTRR support: it won't
> > allow to use the PAT MSR, even if there is no technical reason for
> > that, other than setting up PAT on all cpus the same way (which is a
> > requirement of the processor's cache management) is relying on some
> > MTRR specific code.
> > 
> > Fix all of that by:
> > 
> > - moving the function needed by PAT from MTRR specific code one level
> >   up
> > - adding a PAT indirection layer supporting the 3 cases "no or disabled
> >   PAT", "PAT under kernel control", and "PAT under Xen control"
> > - removing the dependency of PAT on MTRR
>
> Thx for working on this. If you need to respin these patches for one
> reason or another, could you do me a favor and add proper 'Link:' tags
> pointing to all reports about this issue? e.g. like this:
>
>  Link: https://lore.kernel.org/regressions/YnHK1Z3o99eMXsVK@mail-itl/
>
> These tags are considered important by Linus[1] and others, as they
> allow anyone to look into the backstory weeks or years from now. That is
> why they should be placed in cases like this, as
> Documentation/process/submitting-patches.rst and
> Documentation/process/5.Posting.rst explain in more detail. I care
> personally, because these tags make my regression tracking efforts a
> whole lot easier, as they allow my tracking bot 'regzbot' to
> automatically connect reports with patches posted or committed to fix
> tracked regressions.
>
> [1] see for example:
> https://lore.kernel.org/all/CAHk-=wjMmSZzMJ3Xnskdg4+GGz=5p5p+GSYyFBTh0f-DgvdBWg@xxxxxxxxxxxxxx/
> https://lore.kernel.org/all/CAHk-=wgs38ZrfPvy=nOwVkVzjpM3VFU1zobP37Fwd_h9iAD5JQ@xxxxxxxxxxxxxx/
> https://lore.kernel.org/all/CAHk-=wjxzafG-=J8oT30s7upn4RhBs6TX-uVFZ5rME+L5_DoJA@xxxxxxxxxxxxxx/
>
> Ciao, Thorsten (wearing his 'the Linux kernel's regression tracker' hat)
>
> P.S.: As the Linux kernel's regression tracker I deal with a lot of
> reports and sometimes miss something important when writing mails like
> this. If that's the case here, don't hesitate to tell me in a public
> reply, it's in everyone's interest to set the public record straight.
>
> BTW, let me tell regzbot to monitor this thread:
>
> #regzbot ^backmonitor:
> https://lore.kernel.org/regressions/YnHK1Z3o99eMXsVK@mail-itl/

Hi Thorsten,

This appears stalled again and we are now over three months
from the first report of the regression, The only excuse for
ignoring your comments, and other comments on the patches
in this patch series for this long a time is that the patch series
for some reason cannot be considered a true regression. If this is a
regression, then, IMHO, this needs to have a higher priority by the
maintainers, or the maintainers need to explain why this regression
cannot be fixed in a more timely manner. But continued silence
by the maintainers is unacceptable, IMHO. This is especially true
in this case when multiple fixes for the regression have been
identified and the maintainers have not yet clearly explained why
at least a fix, even if temporary, cannot be applied immediately
while we wait for a more comprehensive fix.

At the very least, I would expect Juergen to reply here and say that
he is delayed but does plan to spin up an updated version and include
the necessary links in the new version to facilitate your tracking of
the regression. Why the silence from Juergen here?

Best regards,

Chuck