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

[Chris Wilson]

On Mon, Aug 24, 2015 at 06:04:28PM +0800, Zhiyuan Lv wrote:
> 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.

Wrong. It exposes the guest to a trivial denial-of-service attack. A
smaller GGTT does not actually limit clients (there is greater aperture
pressure and some paths are less likely but an individual client will
function just fine).
 
> 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. 

That is just a bug in your code. Firstly allowing swapout on an object
you still are using, secondly not being able to swapin.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre