From: Pan Xinhui <xinhui@linux.vnet.ibm.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
Christian Borntraeger <borntraeger@de.ibm.com>,
Pan Xinhui <xinhui.pan@linux.vnet.ibm.com>,
linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
virtualization@lists.linux-foundation.org,
linux-s390@vger.kernel.org,
xen-devel-request@lists.xenproject.org, kvm@vger.kernel.org,
benh@kernel.crashing.org, paulus@samba.org, mpe@ellerman.id.au,
mingo@redhat.com, paulmck@linux.vnet.ibm.com,
will deacon <will.deacon@arm.com>,
kernellwp@gmail.com, jgross@suse.com, bsingharora@gmail.com,
Heiko Carstens <heiko.carstens@de.ibm.com>
Subject: Re: [PATCH v3 0/4] implement vcpu preempted check
Date: Fri, 30 Sep 2016 17:31:48 +0800 [thread overview]
Message-ID: <ce64b907-6316-7c36-428b-680d869ef045@linux.vnet.ibm.com> (raw)
In-Reply-To: <f0e8b549-e647-6a01-a93d-9f192adc70c6@redhat.com>
在 2016/9/30 17:08, Paolo Bonzini 写道:
>
>
> On 30/09/2016 10:52, Pan Xinhui wrote:
>>> x86 has no hypervisor support, and I'd like to understand the desired
>>> semantics first, so I don't think it should block this series. In
>>
>> Once a guest do a hypercall or something similar, IOW, there is a
>> kvm_guest_exit. we think this is a lock holder preemption.
>> Adn PPC implement it in this way.
>
> Ok, good.
>
>>> particular, there are at least the following choices:
>>>
>>> 1) exit to userspace (5-10.000 clock cycles best case) counts as
>>> lock holder preemption
>>>
>>> 2) any time the vCPU thread not running counts as lock holder
>>> preemption
>>>
>>> To implement the latter you'd need a hypercall or MSR (at least as
>>> a slow path), because the KVM preempt notifier is only active
>>> during the KVM_RUN ioctl.
>>
>> seems a little expensive. :(
>> How many clock cycles it might cost.
>
> An MSR read is about 1500 clock cycles, but it need not be the fast path
> (e.g. use a bit to check if the CPU is running, if not use the MSR to
> check if the CPU is in userspace but the CPU thread is scheduled). But
> it's not necessary if you are just matching PPC semantics.
>
> Then the simplest thing is to use the kvm_steal_time struct, and add a
> new field to it that replaces pad[0]. You can write a 0 to the flag in
> record_steal_time (not preempted) and a 1 in kvm_arch_vcpu_put
> (preempted). record_steal_time is called before the VM starts running,
> immediately after KVM_RUN and also after every sched_in.
>
> If KVM doesn't implement the flag, it won't touch that field at all. So
> the kernel can write a 0, meaning "not preempted", and not care if the
> hypervisor implements the flag or not: the answer will always be safe.
>
> The pointer to the flag can be placed in a per-cpu u32*, and again if
> the u32* is NULL that means "not preempted".
>
really nice suggestion! That's what I want :)
thanks
xinhui
> Paolo
>
>
>> I am still looking for one shared struct between kvm and guest kernel on
>> x86.
>> and every time kvm_guest_exit/enter called, we store some info in it. So
>> guest kernel can check one vcpu is running or not quickly.
>>
>> thanks
>> xinhui
>>
>>> Paolo
>>>
>>
>
next prev parent reply other threads:[~2016-09-30 9:32 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-07-21 11:45 [PATCH v3 0/4] implement vcpu preempted check Pan Xinhui
2016-07-21 11:45 ` [PATCH v3 1/4] kernel/sched: introduce vcpu preempted check interface Pan Xinhui
2016-07-21 11:45 ` [PATCH v3 2/4] powerpc/spinlock: support vcpu preempted check Pan Xinhui
2016-07-21 11:45 ` [PATCH v3 3/4] locking/osq: Drop the overhead of osq_lock() Pan Xinhui
2016-07-21 11:45 ` [PATCH v3 4/4] kernel/locking: Drop the overhead of {mutex, rwsem}_spin_on_owner Pan Xinhui
2016-09-29 10:10 ` [PATCH v3 0/4] implement vcpu preempted check Peter Zijlstra
2016-09-29 10:23 ` Christian Borntraeger
2016-09-29 10:31 ` Peter Zijlstra
2016-09-30 5:03 ` Pan Xinhui
2016-09-30 6:58 ` Paolo Bonzini
2016-09-30 8:52 ` Pan Xinhui
2016-09-30 9:08 ` Paolo Bonzini
2016-09-30 9:31 ` Pan Xinhui [this message]
2016-09-30 10:44 ` Christian Borntraeger
2016-09-29 10:40 ` Christian Borntraeger
2016-09-29 11:05 ` Christian Borntraeger
2016-09-30 4:00 ` Pan Xinhui
2016-10-05 11:00 ` Christian Borntraeger
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