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

From: Boris Ostrovsky
Date: Mon Dec 14 2015 - 10:58:11 EST


On 12/14/2015 10:35 AM, Roger Pau Monné wrote:
El 14/12/15 a les 16.27, Konrad Rzeszutek Wilk ha escrit:
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.

OK, I then mis-read the hypervisor code, I didn't realize that vcpumask_to_pcpumask() takes into account vcpu_dirty_cpumask.



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.

Perhaps if we have PV aware TLB flush it could do this differently?
Why don't HVM/PVH just uses the HVMOP_flush_tlbs hypercall?

It doesn't take any parameters so it will invalidate TLBs for all VCPUs, which is more than is being asked for. Especially in the case of MMUEXT_INVLPG_MULTI.

(That's in addition to the fact that it currently doesn't work for PVH as it has a test for is_hvm_domain() instead of has_hvm_container_domain()).

-boris
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