linux-kernel - Re: [PATCH] xen/x86/pvh: Use HVM's flush_tlb

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Date:	Tue, 15 Dec 2015 08:03:56 -0700
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Boris Ostrovsky" <boris.ostrovsky@...cle.com>,
	"Konrad Rzeszutek Wilk" <konrad.wilk@...cle.com>
Cc:	<#@...r.us.oracle.com>, <3.14+@...r.us.oracle.com>,
	<david.vrabel@...rix.com>, <xen-devel@...ts.xenproject.org>,
	<linux-kernel@...r.kernel.org>, <stable@...r.kernel.org>
Subject: Re: [PATCH] xen/x86/pvh: Use HVM's flush_tlb_others op

>>> On 15.12.15 at 15:36, <boris.ostrovsky@...cle.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. As long as they're useless if called, I'd still favor making
them inaccessible.

Jan

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