linux-kernel - Re: [Xen-devel] [RFC PATCH] Use vAPIC when doing IPI for PVHVM guests.

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Date:	Thu, 8 Oct 2015 10:30:52 +0100
From:	Andrew Cooper <andrew.cooper3@...rix.com>
To:	Juergen Gross <jgross@...e.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	<boris.ostrovsky@...cle.com>, <joao.m.martins@...cle.com>,
	<david.vrabel@...rix.com>, <dario.faggioli@...rix.com>,
	<xen-devel@...ts.xenproject.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [Xen-devel] [RFC PATCH] Use vAPIC when doing IPI for PVHVM guests.

On 08/10/15 06:05, Juergen Gross wrote:
> On 10/07/2015 10:21 PM, Konrad Rzeszutek Wilk wrote:
>> Hey,
>>
>> I was running some tools in which we would heavily do rescheduling
>> of events - and realized to my surprise that the event channels (and
>> the hypercall) would slow things down. If I used the vAPIC with its
>> IPI support (so no VMEXIT) I got much much better performance.
>>
>> Now this is an RFC because:
>>   1). I hadn't verified from the xentrace  how much less VMEXITS we get.
>>      But I remember Boris's patches and they gave at least 10%.
>>      I think this will get the same performance or even better.
>>
>>   2). I don't know what to do with migration. That is if the guest
>>      migrates to older hardware it needs to recheck this I presume?
>
> Same problem applies to many other features. In case you want to
> migrate to a machine with less features you'd have to mask those
> features in the cpuid data of the domain.

Those leaves in particular are from the HV set rather than the plain
featureset.  One way or another there will be an APIC to use, but those
features are expected to appear/disappear across migrate to indicate
whether hardware assistance is in use or not.

Therefore, they should be resampled and re-acted-upon in the resume path.

~Andrew
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