Re: [Xen-devel] GPU passthrough performance regression in >4GB vms due to XSA-60 changes

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 15.05.14 at 16:56, <tomasz.wroblewski@xxxxxxxxx> wrote:

> On 05/15/2014 04:34 PM, Tomasz Wroblewski wrote:
>>
>> On 05/15/2014 03:39 PM, Tomasz Wroblewski wrote:
>>>
>>> On 05/15/2014 03:23 PM, Jan Beulich wrote:
>>>>>>> On 15.05.14 at 14:10, <tomasz.wroblewski@xxxxxxxxx> wrote:
>>>>> Not really sure why it only affects 64bit vms but I've just noticed 
>>>>> the
>>>>> pci BARs for the card are being relocated by hvmloader as per some 
>>>>> logs:
>>>>>
>>>>> (XEN) HVM3: Relocating guest memory for lowmem MMIO space enabled
>>>>> (XEN) HVM3: Relocating 0xffff pages from 0e0001000 to 14dc00000 for
>>>>> lowmem MMIO hole
>>>>> (XEN) HVM3: Relocating 0x1 pages from 0e0000000 to 15dbff000 for 
>>>>> lowmem
>>>>> MMIO hole
>>>>>
>>>>> So it might be also related to that.
>>>> Indeed it might - what are the (guest) MTRR types for those regions?
>>> It's writeback for both the 32bit and 64bit above ranges.
>> ... however, after a bit more debugging its uncached at the time 
>> hvmloader does the relocation so that's why it ends up like that in 
>> EPT tables. It does go to writeback only soon after. Haven't 
>> pinpointed the exact time point for that yet nor why it's being 
>> updated to writeback, but it seems to be before the guest starts 
>> booting (i.e. still on bios screens).
> ... and after even more I see that the type is uncached at the time the 
> relocation is happening because mtrr is disabled at that time and 
> get_mtrr_type() function exits with uncached value in the first few 
> lines of it. Later when guests enabled MTRR, ept is not updated. So 
> maybe the EPTs should be updated in some way at that time,

Which is what -unstable is now doing.

But the question remains why this region doesn't get marked UC or
WC, but WB.

Jan


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