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

[Boris Ostrovsky]

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.

Jan?

-boris


> Perhaps if we have PV aware TLB flush it could do this differently?
>
> > Signed-off-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
> > Suggested-by: Jan Beulich <jbeulich@xxxxxxxx>
> > Cc: stable@xxxxxxxxxxxxxxx # 3.14+
> > ---
> >   arch/x86/xen/mmu.c |    9 ++-------
> >   1 files changed, 2 insertions(+), 7 deletions(-)
> >
> > diff --git a/arch/x86/xen/mmu.c b/arch/x86/xen/mmu.c
> > index 9c479fe..9ed7eed 100644
> > --- a/arch/x86/xen/mmu.c
> > +++ b/arch/x86/xen/mmu.c
> > @@ -2495,14 +2495,9 @@ void __init xen_init_mmu_ops(void)
> >   {
> >   	x86_init.paging.pagetable_init = xen_pagetable_init;
> >   
> > -	/* Optimization - we can use the HVM one but it has no idea which
> > -	 * VCPUs are descheduled - which means that it will needlessly IPI
> > -	 * them. Xen knows so let it do the job.
> > -	 */
> > -	if (xen_feature(XENFEAT_auto_translated_physmap)) {
> > -		pv_mmu_ops.flush_tlb_others = xen_flush_tlb_others;
> > +	if (xen_feature(XENFEAT_auto_translated_physmap))
> >   		return;
> > -	}
> > +
> >   	pv_mmu_ops = xen_mmu_ops;
> >   
> >   	memset(dummy_mapping, 0xff, PAGE_SIZE);
> > --
> > 1.7.1

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