Hi All,
I have spent the last few days attempting to track down the cause of the
performance degradation when ntp is enabled on AMD-Vi systems and VGA
pass through.
A few hours ago I identified why Intel is working and AMD is not and
found a
work around that fixes the problem, I am now seeing near native
performance
with ntp enabled on an AMD Ryzen platform.
https://www.3dmark.com/3dm/22878932
https://www.3dmark.com/3dm/22879024
Since my understanding of hardware at this level on this platform is
limited it
needs a guru to take this and implement a fix or perhaps even a quirk
that can
be enabled to work around this problem.
In svm.c, by just changing the line in `init_vmcb` that reads:
save->g_pat = svm->vcpu.arch.pat;
To:
save->g_pat = 0x0606060606060606;
The problem is resolved. From what I understand this is setting a MTTR
value
that enables Write Back (WB). Since finding this gem I found the
following patch
set also:
https://patchwork.kernel.org/patch/6748441/
This doesn't fix the problem but it seems to be heading in the right
direction.
-Geoff
