Re: [RFC][PATCH] KVM: SVM: Sync g_pat with guest-written PAT value

["Radim Krčmář" <rkrcmar@redhat.com>]

2015-04-20 19:45+0200, Jan Kiszka:
> On 2015-04-20 19:37, Jan Kiszka wrote:
> > On 2015-04-20 19:33, Radim Krčmář wrote:
> >> 2015-04-20 19:21+0200, Jan Kiszka:
> >>> On 2015-04-20 19:16, Radim Krčmář wrote:
> >>>> 2015-04-20 18:14+0200, Radim Krčmář:
> >>>>> Tested-by: Radim Krčmář <rkrcmar@redhat.com>
> >>>>
> >>>> Uncached accesses were roughly 20x slower.
> >>>> In case anyone wanted to reproduce, I used this as a kvm-unit-test:
> >>>>
> >>>> ---
> >> | [code]
> >>>
> >>> Great, thanks. Will you push it to the unit tests? Could raise
> >>> motivations to fix the !NPT/EPT case.
> >>
> >> It can't be included in `run_tests.sh`, because we intenionally ignore
> >> PAT for normal RAM on VMX and the test does "fail" ...
> > 
> > That ignoring is encoded into the EPT?

Yes, it's the VMX_EPT_IPAT_BIT.

> And do you also know why is it ignored on Intel? Side effects on the host?

I think it is an optimization exclusive to Intel.
We know that the other side is not real hardware, which could avoid CPU
caches when accessing memory, so there is little reason to slow the
guest down.

> >                                        Hmm... Maybe we can create a
> > ivshmem device and use that as test target.

Good idea, thanks.
(Haven't used it yet, so its parts might be able to do what is needed
 without creating a dependency on the whole ivshmem system.)