Re: [Xen-devel] XEN MTRR

["aorchis@xxxxxxxxx" <aorchis@xxxxxxxxx>]

On Wed, Jun 6, 2012 at 2:17 AM, Konrad Rzeszutek Wilk
<konrad.wilk@xxxxxxxxxx> wrote:
> On Sun, Jun 03, 2012 at 05:31:32PM +1000, aorchis@xxxxxxxxx wrote:
>> Hi Jeremy and Konrad,
>
> CC-ing xen-devel.
>
>>
>> Basically the driver NVIDIA provided is a binary blob and recent
>> versions does not work with the PAT layout of XEN so it falls back to
>> MTRR to provide write combining (please correct me if I'm wrong).
>
> OK? Which is still OK. Are you using a v3.4 kernel with an up-to-date
> NVidia driver? I've had reports that it works OK.

I briefly tried kernel 3.4 to see if the problem is fixed but it's not.
I used v3.4 kernel with NVIDIA driver v295.49 and a beta version
(v302) but both didn't work.

When the nvidia module is loaded, it prints out an error message:
"NVRM: PAT configuration unsupported, falling back to MTRRs."

When I launch Xorg, the screen turns blank then the monitors powered down and my
box hard crashed, I had to hold the power button to turn it off.

This only happens in dom0 under XEN, if I run my dom0 by itself then
the nvidia module loads fine.

>> However there is no MTRR support on XEN so the driver hard crashed my
>> machine (I can't ssh into the box anymore).
>>
>> Moreover, there is problem with the open source driver 'nouveau' for
>> NVIDIA card  (also has something to do with PAT layout of XEN) which
>> causes memory corruption.
>
> Huh? Can you point me to a bugzilla please? There was a corruption
> issue where you can pass in 'nopat' on the command line.

Yes, that is the issue I was referring to, I had to pass "nopat" to
GRUB in order
to fix it.

>>
>> I found several patches for XEN which supposedly provide basic MTRR
>> support for XEN however there is still no /proc/mtrr. Jeremy, can you
>> tell me if you had been able to get /proc/mtrr on XEN dom0?
>
>>
>> Thanks for your time.
>>
>> Damien.
>>
>>
>> On Sun, Jun 3, 2012 at 5:37 AM, Jeremy Fitzhardinge <jeremy@xxxxxxxx> wrote:
>> > On 06/02/2012 03:13 AM, aorchis@xxxxxxxxx wrote:
>> >> Hi Jeremy,
>> >>
>> >> Is there any way I can get back MTRR support in XEN in 3.0 kernel? To
>> >> make a long story short, NVIDIA binary driver rejects PAT in XEN and
>> >> it falls back to using MTRR but MTRR in XEN was taken out a long time
>> >> ago so now there's no way to get the NVIDIA binary blob running under
>> >> a linux XEN dom0. I was about to tear my hair out looking for
>> >> solutions high and low.
>> >
>> > Hi!
>> >
>> > Firstly, Konrad is probably the person you should send this to these
>> > days, since I'm not managing to get much Xen stuff done.
>> >
>> > Secondly, hm.  Unfortunately, the changes we did have to integrate Xen's
>> > MTRR machinery with Linux have been solidly rejected by the upstream
>> > maintainers several times, so I think its unlikely that they will ever
>> > make it into the mainline kernel.  And it doesn't seem to have really
>> > made a difference because PAT does subsume MTRR for at least all the in
>> > kernel users, as far as I know.
>> >
>> > What do you mean by "[the] NVIDIA binary driver rejects PAT in XEN and
>> > it falls back to using MTRR"?  Why does the Nvidia driver reject PAT?
>> > Perhaps addressing that would be a more profitable way of getting this
>> > working.  In the past we've talked about changing Xen's PAT mapping to
>> > match the kernel's (or make it configurable), but for now we're
>> > remapping between the PAT schemes in the pte pvops.  If the NVIDIA
>> > driver is using that mechanism to set ptes (as it must to get anywhere
>> > in a pvops kernel), then it should be fine with the remapping.  Or its
>> > possible they're having problems with reading a pte back and mapping
>> > from Xen->Linux PAT formats, which is a problem some of the in-kernel
>> > drivers also had.  Konrad, how did that turn out in the end?
>
> Attic. I've turned it off since we had corruption issues (the WC didn't
> turn back into WB b/c of page_attr using the pte_flag instead of pte_var).
> Peter was talking about some software PAT lookup code but I hadn't
> focused on that. There is also some performance numbers to run and collect.
>

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