Messages in this thread | | | Subject | Re: [PATCH] xen/x86/pvh: Use HVM's flush_tlb_others op | From | Boris Ostrovsky <> | Date | Tue, 15 Dec 2015 10:14:49 -0500 |
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On 12/15/2015 10:03 AM, Jan Beulich wrote: >>>> On 15.12.15 at 15:36, <boris.ostrovsky@oracle.com> 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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