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

[Tomasz Wroblewski <tomasz.wroblewski@xxxxxxxxx>]

On 05/15/2014 06:07 PM, Jan Beulich wrote:
> > > > 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.
Right.
> But the question remains why this region doesn't get marked UC or
> WC, but WB.
The region doesn't seem to be marked in any way in mtrr so it just goes 
off the default type for that mtrr ((struct mtrr_state*)->def_type) 
which seems to be WB.
> Jan

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