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

On 05/19/2014 01:07 PM, Jan Beulich wrote:
On 19.05.14 at 12:47, <tomasz.wroblewski@xxxxxxxxx> wrote:
On 05/19/2014 12:38 PM, Jan Beulich wrote:
So perhaps time for sending complete logs, plus suitable information
from inside the guest of how things (RAM, MMIO, MTRRs) end up being
set up?
Could be, though please read the explanation I came up in the other post
whether its enough, I think it makes sense... 64bit guest BARs are
indeed not in use (confirmed from guest). MTRR is setup such that only
the low region is UC, which is correct.
Yes, that's a very sensible theory, which - as just said in the other
reply - can be easily verified.

But the RAM relocation code causes the caching on relocated region to be
UC instead of WB due to the timing (very early, MTRR disabled) at which
it runs, which is incorrect. I am thinking enabling MTRR during that
relocation would probably fix it on 4.3
Except that this is a chicken and egg problem then: In order to
populate the variable range MTRRs, the BAR assignment (and hence
the prerequisite RAM relocation) need to be done already.
I am not sure; looking at hvmloader code, wouldn't it be possible to calculate the BAR locations first, then update the MTRR var ranges and enable it, and only then actually write the BAR registers (from precalculated info)? Presumably it's only the write part which needs to be done after relocation as it causes qemu to setup mmio etc.

What we
might be able to do (after careful evaluation whether backporting
what we have on -unstable wouldn't be the better option, which
first of all implies you'd need to test things there) is enable MTRRs
with WB default, but without any variable ranges set up _before_
doing the RAM relocation/BAR assignment. The main problem to solve
there, however, would still be to make sure the EPT entries get
re-created for the regions that we don't want to be WB (after the
variable ranges got set up). That, I'm afraid, would still lead us to
considerations on backporting at least some of the changes from
Yeah I gave about a day of effort to port us onto unstable and test there but it sadly looks to be a bigger job, so leaving that as a last resort (though planning to spend couple more days on it soon).


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