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

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 07/28/2016 06:54 AM, Andrew Cooper wrote:
> 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.

Is this really safe to do? Can't a guest decide to disable cache to
avoid having to deal with coherency in SW?

As far as Intel vs AMD implementation in Xen, we have vmx_handle_cd()
but no corresponding SVM code. Could it be that we need to set gPAT, for
example?

-boris


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