From: Christian Borntraeger <borntraeger@de.ibm.com>
To: "Radim Krčmář" <rkrcmar@redhat.com>,
"David Hildenbrand" <david@redhat.com>
Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
Paolo Bonzini <pbonzini@redhat.com>,
Andrew Jones <drjones@redhat.com>,
Marc Zyngier <marc.zyngier@arm.com>,
Cornelia Huck <cornelia.huck@de.ibm.com>,
James Hogan <james.hogan@imgtec.com>,
Paul Mackerras <paulus@ozlabs.org>,
Christoffer Dall <christoffer.dall@linaro.org>
Subject: Re: [PATCH 4/5] KVM: add __kvm_request_needs_mb
Date: Wed, 22 Feb 2017 20:57:09 +0100 [thread overview]
Message-ID: <8960b809-0faa-58e5-4839-b28a09f161d6@de.ibm.com> (raw)
In-Reply-To: <20170222151704.GA3856@potion>
On 02/22/2017 04:17 PM, Radim Krčmář wrote:
>
[...]
> while (vcpu->arch.sie_block->prog0c & PROG_IN_SIE)
> cpu_relax();
> }
> And out of curiosity -- how many cycles does this loop usually take?
A quick hack indicates something between 3 and 700ns.
>> 2. Remote requests that don't need a sync
>>
>> E.g. KVM_REQ_ENABLE_IBS doesn't strictly need it, while
>> KVM_REQ_DISABLE_IBS does.
>
> A usual KVM request would kick the VCPU out of nested virt as well.
> Shouldn't it be done for these as well?
A common code function probably should. For some of the cases (again
prefix page handling) we do not need it. For example if we unmap
the guest prefix page, but guest^2 is running this causes no trouble
as long as we handle the request before reentering guest^1. So
not an easy answer.
>
>> 3. local requests
>>
>> E.g. KVM_REQ_TLB_FLUSH from kvm_s390_set_prefix()
>>
>>
>> Of course, having a unified interface would be better.
>>
>> /* set the request and kick the CPU out of guest mode */
>> kvm_set_request(req, vcpu);
>>
>> /* set the request, kick the CPU out of guest mode, wait until guest
>> mode has been left and make sure the request will be handled before
>> reentering guest mode */
>> kvm_set_sync_request(req, vcpu);
>
> Sounds good, I'll also add
>
> kvm_set_self_request(req, vcpu);
>
>> Same maybe even for multiple VCPUs (as there are then ways to speed it
>> up, e.g. first kick all, then wait for all)
>>
>> This would require arch specific callbacks to
>> 1. pre announce the request (e.g. set PROG_REQUEST on s390x)
>> 2. kick the cpu (e.g. CPUSTAT_STOP_INT and later
>> kvm_s390_vsie_kick(vcpu) on s390x)
>> 3. check if still executing the guest (e.g. PROG_IN_SIE on s390x)
>>
>> This would only make sense if there are other use cases for sync
>> requests. At least I remember that Power also has a faster way for
>> kicking VCPUs, not involving SMP rescheds. I can't judge if this is a
>> s390x only thing and is better be left as is :)
>>
>> At least vcpu_kick() could be quite easily made to work on s390x.
>>
>> Radim, are there also other users that need something like sync requests?
>
> I think that ARM has a similar need when updating vgic, but relies on an
> asumption that VCPUs are going to be out after kicking them with
> kvm_make_all_cpus_request().
> (vgic_change_active_prepare in virt/kvm/arm/vgic/vgic-mmio.c)
>
> Having synchronous requests in a common API should probably wait for the
> completion of the request, not just for the kick, which would make race
> handling simpler.
This would be problematic for our prefix page handling due to locking.
next prev parent reply other threads:[~2017-02-22 22:41 UTC|newest]
Thread overview: 31+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-02-16 16:04 [PATCH 0/5] KVM: rename and extend vcpu->requests API Radim Krčmář
2017-02-16 16:04 ` [PATCH 1/5] KVM: change API for requests to match bit operations Radim Krčmář
2017-02-17 9:30 ` Cornelia Huck
2017-02-17 9:49 ` Andrew Jones
2017-02-17 9:52 ` Cornelia Huck
2017-02-17 15:01 ` Radim Krčmář
2017-02-16 16:04 ` [PATCH 2/5] KVM: add KVM request variants without barrier Radim Krčmář
2017-02-23 10:57 ` Paolo Bonzini
2017-02-23 15:50 ` Radim Krčmář
2017-02-16 16:04 ` [PATCH 3/5] KVM: optimize kvm_make_all_cpus_request Radim Krčmář
2017-02-16 16:04 ` [PATCH 4/5] KVM: add __kvm_request_needs_mb Radim Krčmář
2017-02-16 19:49 ` David Hildenbrand
2017-02-16 21:31 ` Radim Krčmář
2017-02-17 8:46 ` Christian Borntraeger
2017-02-17 10:13 ` David Hildenbrand
2017-02-17 10:19 ` Christian Borntraeger
2017-02-17 11:28 ` Christian Borntraeger
2017-02-22 15:17 ` Radim Krčmář
2017-02-22 19:23 ` Christian Borntraeger
2017-02-23 15:43 ` Radim Krčmář
2017-02-22 19:57 ` Christian Borntraeger [this message]
2017-02-23 10:20 ` David Hildenbrand
2017-02-23 15:39 ` Radim Krčmář
2017-02-24 11:34 ` Christoffer Dall
2017-02-24 12:46 ` Andrew Jones
2017-02-23 11:01 ` Paolo Bonzini
2017-02-23 15:52 ` Radim Krčmář
2017-02-16 16:04 ` [PATCH 5/5] KVM: add kvm_request_pending Radim Krčmář
2017-02-16 19:50 ` David Hildenbrand
2017-02-17 9:51 ` Andrew Jones
2017-02-17 14:59 ` Radim Krčmář
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