[vfio-users] BSOD occuring with 1 game in VM

[Ivan Volosyuk]

Thank you for the investigation. Now I have a solid reason to postpone the
Win10 upgrade.

On Sat, May 14, 2016, 11:21 AM Abdulla Bubshait <darkstego at gmail.com> wrote:

> So for the people having problem with Heroes of the Storm or
> StarCraft2 on Windows 10,
> here is what I have been able to piece together.
>
> Those two games use a ton of branch tracing code. Specifically, they
> enable and access
> LBR registers (part of the Intel MSRs). The windows 10 code portion
> for these games
> seems to include a ton of LBR reads. Since these branch traces are
> mainly for code
> debugging purposes I personally believe that they were used to debug
> win10 code for
> these games and were forgotten. LBRs themselves don't hurt performance
> much, although
> having all those needless register reads can't be that beneficial.
>
> KVM does not support LBR. So without the ignore_msrs the game would
> just crash. But
> even if KVM simply returned 0 that is still a relatively expensive
> process when compared
> to a simple register read in native. Even if KVM were to implement LBR
> that would only
> make things worse because any implementation would cost more than a
> simple return 0.
>
> With my limited knowledge of how KVM works I do not see how one can make a
> host
> respond to guests hammering LBR requests without taking a performance
> hit, even if
> they don't give the correct response.
>
> So it seems the only way to fix this performance issue is to have
> Blizzard take out the
> offending code in an upcoming patch. I feel the code was left in there
> in error as at the
> end of the day it's main purpose is debugging and shouldn't be active
> in a game where
> performance really matters. The question is how likely it is that
> Blizzard will actually fix
> this problem seeing as it really effects only a tiny subset of their user
> base.
>
> I really wish I had a proper fix to the problem, but it seems that
> getting these games to
> run in VM preperly means avoiding win10.
>
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