[Intel-gfx] About the iGVT-g's requirement to pin guest contexts in VM

[Zhiyuan Lv]

Hi Chris,

On Thu, Aug 20, 2015 at 09:36:00AM +0100, Chris Wilson wrote:
> On Thu, Aug 20, 2015 at 03:45:21PM +0800, Zhiyuan Lv wrote:
> > Intel GVT-g will perform EXECLIST context shadowing and ring buffer
> > shadowing. The shadow copy is created when guest creates a context.
> > If a context changes its LRCA address, the hypervisor is hard to know
> > whether it is a new context or not. We always pin context objects to
> > global GTT to make life easier.
> 
> Nak. Please explain why we need to workaround a bug in the host. We
> cannot pin the context as that breaks userspace (e.g. synmark) who can
> and will try to use more contexts than we have room.

Could you have a look at below reasons and kindly give us your inputs?

1, Due to the GGTT partitioning, the global graphics memory available
inside virtual machines is much smaller than native case. We cannot
support some graphics memory intensive workloads anyway. So it looks
affordable to just pin contexts which do not take much GGTT.

2, Our hypervisor needs to change i915 guest context in the shadow
context implementation. That part will be tricky if the context is not
always pinned. One scenario is that when a context finishes running,
we need to copy shadow context, which has been updated by hardware, to
guest context. The hypervisor knows context finishing by context
interrupt, but that time shrinker may have unpin the context and its
backing storage may have been swap-out. Such copy may fail. 

Look forward to your comments. Thanks!

Regards,
-Zhiyuan


> -Chris
> 
> -- 
> Chris Wilson, Intel Open Source Technology Centre