From: Marc Zyngier <maz@kernel.org>
To: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Will Deacon <will@kernel.org>,
Suleiman Souhlal <suleiman@google.com>,
Joel Fernandes <joelaf@google.com>,
linux-arm-kernel@lists.infradead.org,
kvmarm@lists.cs.columbia.edu, linux-kernel@vger.kernel.org,
virtualization@lists.linux-foundation.org
Subject: Re: [PATCHv2 4/4] arm64: add host pv-vcpu-state support
Date: Wed, 21 Jul 2021 10:10:59 +0100 [thread overview]
Message-ID: <87fsw82frw.wl-maz@kernel.org> (raw)
In-Reply-To: <YPd1Q1ppmKng67tk@google.com>
On Wed, 21 Jul 2021 02:15:47 +0100,
Sergey Senozhatsky <senozhatsky@chromium.org> wrote:
>
> On (21/07/12 17:24), Marc Zyngier wrote:
> > >
> > > void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
> > > {
> > > + kvm_update_vcpu_preempted(vcpu, true);
> >
> > This doesn't look right. With this, you are now telling the guest that
> > a vcpu that is blocked on WFI is preempted. This really isn't the
> > case, as it has voluntarily entered a low-power mode while waiting for
> > an interrupt. Indeed, the vcpu isn't running. A physical CPU wouldn't
> > be running either.
>
> I suppose you are talking about kvm_vcpu_block().
kvm_vcpu_block() is how things are implemented. WFI is the instruction
I'm concerned about.
> Well, it checks kvm_vcpu_check_block() but then it simply schedule()
> out the vcpu process, which does look like "the vcpu is
> preempted". Once we sched_in() that vcpu process again we mark it as
> non-preempted, even though it remains in kvm wfx handler. Why isn't
> it right?
Because the vcpu hasn't been "preempted". It has *voluntarily* gone
into a low-power mode, and how KVM implements this "low-power mode" is
none of the guest's business. This is exactly the same behaviour that
you will have on bare metal. From a Linux guest perspective, the vcpu
is *idle*, not doing anything, and only waiting for an interrupt to
start executing again.
This is a fundamentally different concept from preempting a vcpu
because its time-slice is up. In this second case, you can indeed
mitigate things by exposing steal time and preemption status as you
break the illusion of a machine that is completely controlled by the
guest.
If the "reched on interrupt delivery while blocked on WFI" is too slow
for you, then *that* is the thing that needs addressing. Feeding extra
state to the guest doesn't help.
> Another call path is iret:
>
> <iret>
> __schedule()
> context_switch()
> prepare_task_switch()
> fire_sched_in_preempt_notifiers()
> kvm_sched_out()
> kvm_arch_vcpu_put()
I'm not sure how a x86 concept is relevant here.
Thanks,
M.
--
Without deviation from the norm, progress is not possible.
prev parent reply other threads:[~2021-07-21 9:21 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-07-09 4:37 [PATCHv2 0/4] arm64:kvm: teach guest sched that VCPUs can be preempted Sergey Senozhatsky
2021-07-09 4:37 ` [PATCHv2 1/4] arm64: smccc: Add SMCCC pv-vcpu-state function call IDs Sergey Senozhatsky
2021-07-12 14:22 ` Marc Zyngier
2021-07-09 4:37 ` [PATCHv2 2/4] arm64: add guest pvstate support Sergey Senozhatsky
2021-07-09 7:39 ` David Edmondson
2021-07-09 7:52 ` Sergey Senozhatsky
2021-07-09 18:58 ` Joel Fernandes
2021-07-09 21:53 ` Sergey Senozhatsky
2021-07-11 16:58 ` Joel Fernandes
2021-07-12 15:42 ` Marc Zyngier
2021-07-21 2:05 ` Sergey Senozhatsky
2021-07-21 8:22 ` Marc Zyngier
2021-07-21 8:47 ` Sergey Senozhatsky
2021-07-21 10:16 ` Marc Zyngier
2021-07-09 4:37 ` [PATCHv2 3/4] arm64: do not use dummy vcpu_is_preempted() Sergey Senozhatsky
2021-07-12 15:47 ` Marc Zyngier
2021-07-21 2:06 ` Sergey Senozhatsky
2021-07-09 4:37 ` [PATCHv2 4/4] arm64: add host pv-vcpu-state support Sergey Senozhatsky
2021-07-12 16:24 ` Marc Zyngier
2021-07-20 18:44 ` Joel Fernandes
2021-07-21 8:40 ` Marc Zyngier
2021-07-21 10:38 ` Sergey Senozhatsky
2021-07-21 11:08 ` Marc Zyngier
2021-07-21 1:15 ` Sergey Senozhatsky
2021-07-21 9:10 ` Marc Zyngier [this message]
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