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

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Message-ID: <566EE1C4.4080204@citrix.com>
Date:	Mon, 14 Dec 2015 16:35:32 +0100
From:	Roger Pau Monné <roger.pau@...rix.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>
CC:	<3.14+@...r.us.oracle.com>, <linux-kernel@...r.kernel.org>,
	<stable@...r.kernel.org>, <david.vrabel@...rix.com>,
	<jbeulich@...e.com>, <xen-devel@...ts.xenproject.org>,
	<#@...r.us.oracle.com>
Subject: Re: [Xen-devel] [PATCH] xen/x86/pvh: Use HVM's flush_tlb_others op

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.
> 
>>
>> 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?

Roger.

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