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

On 12/15/2015 10:24 AM, Jan Beulich wrote:
On 15.12.15 at 16:14, <boris.ostrovsky@xxxxxxxxxx> wrote:
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
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
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.
I don't follow - an INVLPG done by the hypervisor won't do any
flushing for a HVM guest.

I thought that this would be done with VPID of intended VCPU still loaded and so INVLPG would flush guest's address?


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