linux-kernel - Re: [PART1 RFC v2 10/10] svm: Manage vcpu load/unload when enable AVIC

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Message-ID: <20160310140141.GA23501@potion.brq.redhat.com>
Date:	Thu, 10 Mar 2016 15:01:42 +0100
From:	Radim Krčmář <rkrcmar@...hat.com>
To:	Suravee Suthikulpanit <Suravee.Suthikulpanit@....com>
Cc:	pbonzini@...hat.com, joro@...tes.org, bp@...en8.de,
	gleb@...nel.org, alex.williamson@...hat.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, wei@...hat.com,
	sherry.hurwitz@....com
Subject: Re: [PART1 RFC v2 10/10] svm: Manage vcpu load/unload when enable
 AVIC

2016-03-09 22:46+0100, Radim Krčmář:
> 2016-03-04 14:46-0600, Suravee Suthikulpanit:
>> From: Suravee Suthikulpanit <suravee.suthikulpanit@....com>
>> 
>> When a vcpu is loaded/unloaded to a physical core, we need to update
>> information in the Physical APIC-ID table accordingly.
>> 
>> Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@....com>
>> ---
>> +static int avic_vcpu_load(struct kvm_vcpu *vcpu, int cpu, bool is_load)
> 
> This function does a lot and there is only one thing that must be done
> in svm_vcpu_load:  change host physical APIC ID if the CPU changed.
> The rest can be done elsewhere:
>  - is_running when blocking.

Well, we haven't reached an agreement on is_running yet.  The situation:
if we don't unset vcpu1.is_running when vcpu1 is scheduled out and vcpu2
gets scheduled on vcpu1's physical core, then vcpu2 would receive a
doorbell intended to vcpu1.

We'd like to keep is_running set when there is no reason to vmexit, but
not if a guest can negatively affect other guests.
How does receiving a stray doorbell affect the performance?

Thanks.

(Toggling is_running on load/put is definitely safer, so it's a good
 choice for first version.)