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

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Date:	Tue, 15 Dec 2015 09:36:04 -0500
From:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>, jbeulich@...e.com
Cc:	david.vrabel@...rix.com, xen-devel@...ts.xenproject.org,
	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	#@...r.us.oracle.com, 3.14+@...r.us.oracle.com
Subject: Re: [PATCH] xen/x86/pvh: Use HVM's flush_tlb_others op

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@...cle.com>
>> Suggested-by: Jan Beulich <jbeulich@...e.com>
>> Cc: stable@...r.kernel.org # 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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