Re: [Xen-devel] OVMF very slow on AMD

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On Thu, Jul 28, 2016 at 11:18 AM, Anthony PERARD
<anthony.perard@xxxxxxxxxx> wrote:
> On Wed, Jul 27, 2016 at 03:45:23PM -0400, Boris Ostrovsky wrote:
>> On 07/27/2016 07:35 AM, Anthony PERARD wrote:
>> > On Wed, Jul 27, 2016 at 12:08:04PM +0100, Anthony PERARD wrote:
>> >> I can try to describe how OVMF is setting up the memory.
>> > From the start of the day:
>> > setup gdt
>> > cr0 = 0x40000023
>>
>> I think this is slightly odd, with bit 30 (cache disable) set. I'd
>> suspect that this would affect both Intel and AMD though.
>>
>> Can you try clearing this bit?
>
> That works...
>
> I wonder why it does not appear to affect Intel or KVM.

Are those bits hard-coded, or are they set based on the hardware
that's available?

Is it possible that the particular combination of CPUID bits presented
by Xen on AMD are causing a different value to be written?

Or is it possible that the cache disable bit is being ignored (by Xen)
on Intel and KVM?

 -George

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