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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 28/07/16 11:43, George Dunlap wrote:
> 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?

If a guest has no hardware, then it has no reason to actually disable
caches.  We should have logic to catch this an avoid actually disabling
caches when the guest asks for it.

~Andrew

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