Re: [PATCH v2] KVM: SVM: Sync g_pat with guest-written PAT value

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2015-04-21 13:09, Paolo Bonzini wrote:
> 
> 
> On 20/04/2015 19:25, Jan Kiszka wrote:
>> When hardware supports the g_pat VMCB field, we can use it for emulating
>> the PAT configuration that the guest configures by writing to the
>> corresponding MSR.
>>
>> Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
> 
> I'm not sure about this.  The problem is that, unlike Intel, AMD has no
> way for the host to force its PAT value and ignore the guest's.  I'm
> worried about potential performance problems in the guest.

I think the guest needs to get what it requests - see my remark in
http://thread.gmane.org/gmane.comp.emulators.kvm.devel/135271.

> 
> This is not as bad as on ARM, because the guest cannot disable the cache

You mean AMD, I guess.

> snooping protocol and thus cache coherency is guaranteed (see tables
> 7-10 and 15-20 in the AMD docs), but still I think I'd prefer having
> some knob (module parameter) to enable/disable gPAT.  It's okay to make
> it enabled by default.

I still don't get the scenario where we want to override the guest
settings. Maybe you can help out - would be valuable for the reasoning
in code or commit logs as well.

Jan

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