Re: [PATCH] xen/x86/pvh: Use HVM's flush_tlb_others op

[Boris Ostrovsky]

On 12/15/2015 10:03 AM, Jan Beulich wrote:
> > > > On 15.12.15 at 15:36, <boris.ostrovsky@xxxxxxxxxx> wrote:
> > On 12/14/2015 10:27 AM, Konrad Rzeszutek Wilk wrote:
> > > On Sat, Dec 12, 2015 at 07:25:55PM -0500, Boris Ostrovsky wrote:
> > > > Using MMUEXT_TLB_FLUSH_MULTI doesn't buy us much since the hypervisor
> > > > will likely perform same IPIs as would have the guest.
> > > But if the VCPU is asleep, doing it via the hypervisor will save us waking
> > > up the guest VCPU, sending an IPI - just to do an TLB flush
> > > of that CPU. Which is pointless as the CPU hadn't been running the
> > > guest in the first place.
> > >
> > > > More importantly, using MMUEXT_INVLPG_MULTI may not to invalidate the
> > > > guest's address on remote CPU (when, for example, VCPU from another
> > > > guest
> > > > is running there).
> > > Right, so the hypervisor won't even send an IPI there.
> > >
> > > But if you do it via the normal guest IPI mechanism (which are opaque
> > > to the hypervisor) you and up scheduling the guest VCPU to do
> > > send an hypervisor callback. And the callback will go the IPI routine
> > > which will do an TLB flush. Not necessary.
> > >
> > > This is all in case of oversubscription of course. In the case where
> > > we are fine on vCPU resources it does not matter.
> >
> > So then should we keep these two operations (MMUEXT_INVLPG_MULTI and
> > MMUEXT_TLB_FLUSH_MULT) available to HVM/PVH guests? If the guest's VCPU
> > is not running then TLBs must have been flushed.
> While I followed the discussion, it didn't become clear to me what
> uses these are for HVM guests considering the separate address
> spaces.

To avoid unnecessary IPIs to VCPUs that are not currently scheduled (my 
mistake was that I didn't realize that IPIs to those pCPUs will be 
filtered out by the hypervisor).

> As long as they're useless if called, I'd still favor making
> them inaccessible.


VCPUs that are scheduled will receive the required flush requests.

-boris


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