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From: Will Deacon <will@kernel.org>
To: Shameerali Kolothum Thodi <shameerali.kolothum.thodi@huawei.com>
Cc: "linux-arm-kernel@lists.infradead.org" 
	<linux-arm-kernel@lists.infradead.org>,
	"kvmarm@lists.cs.columbia.edu" <kvmarm@lists.cs.columbia.edu>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"maz@kernel.org" <maz@kernel.org>,
	"catalin.marinas@arm.com" <catalin.marinas@arm.com>,
	"james.morse@arm.com" <james.morse@arm.com>,
	"julien.thierry.kdev@gmail.com" <julien.thierry.kdev@gmail.com>,
	"suzuki.poulose@arm.com" <suzuki.poulose@arm.com>,
	"jean-philippe@linaro.org" <jean-philippe@linaro.org>,
	"Alexandru.Elisei@arm.com" <Alexandru.Elisei@arm.com>,
	"qperret@google.com" <qperret@google.com>,
	Linuxarm <linuxarm@huawei.com>
Subject: Re: [PATCH v3 4/4] KVM: arm64: Clear active_vmids on vCPU schedule out
Date: Tue, 3 Aug 2021 16:30:37 +0100	[thread overview]
Message-ID: <20210803153036.GA31125@willie-the-truck> (raw)
In-Reply-To: <ee2863107d614ef8a36006b5aa912eca@huawei.com>

On Tue, Aug 03, 2021 at 12:55:25PM +0000, Shameerali Kolothum Thodi wrote:
> > > diff --git a/arch/arm64/kvm/vmid.c b/arch/arm64/kvm/vmid.c
> > > index 5584e84aed95..5fd51f5445c1 100644
> > > --- a/arch/arm64/kvm/vmid.c
> > > +++ b/arch/arm64/kvm/vmid.c
> > > @@ -116,6 +116,12 @@ static u64 new_vmid(struct kvm_vmid
> > *kvm_vmid)
> > >  	return idx2vmid(vmid) | generation;
> > >  }
> > >
> > > +/* Call with preemption disabled */
> > > +void kvm_arm_vmid_clear_active(void)
> > > +{
> > > +	atomic64_set(this_cpu_ptr(&active_vmids), 0);
> > > +}
> > 
> > I think this is very broken, as it will force everybody to take the
> > slow-path when they see an active_vmid of 0.
> 
> Yes. I have seen that happening in my test setup.

Why didn't you say so?!

> > It also doesn't solve the issue I mentioned before, as an active_vmid of 0
> > means that the reserved vmid is preserved.
> > 
> > Needs more thought...
> 
> How about we clear all the active_vmids in kvm_arch_free_vm() if it
> matches the kvm_vmid->id ? But we may have to hold the lock 
> there

I think we have to be really careful not to run into the "suspended
animation" problem described in ae120d9edfe9 ("ARM: 7767/1: let the ASID
allocator handle suspended animation") if we go down this road.

Maybe something along the lines of:

ROLLOVER

  * Take lock
  * Inc generation
    => This will force everybody down the slow path
  * Record active VMIDs
  * Broadcast TLBI
    => Only active VMIDs can be dirty
    => Reserve active VMIDs and mark as allocated

VCPU SCHED IN

  * Set active VMID
  * Check generation
  * If mismatch then:
        * Take lock
        * Try to match a reserved VMID
        * If no reserved VMID, allocate new

VCPU SCHED OUT

  * Clear active VMID

but I'm not daft enough to think I got it right first time. I think it
needs both implementing *and* modelling in TLA+ before we merge it!

Will

  reply	other threads:[~2021-08-03 15:30 UTC|newest]

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-29 10:40 [PATCH v3 0/4] kvm/arm: New VMID allocator based on asid Shameer Kolothum
2021-07-29 10:40 ` [PATCH v3 1/4] KVM: arm64: Introduce a new VMID allocator for KVM Shameer Kolothum
2021-08-03 11:38   ` Will Deacon
2021-08-03 12:12     ` Shameerali Kolothum Thodi
2021-07-29 10:40 ` [PATCH v3 2/4] KVM: arm64: Make VMID bits accessible outside of allocator Shameer Kolothum
2021-07-29 10:40 ` [PATCH v3 3/4] KVM: arm64: Align the VMID allocation with the arm64 ASID one Shameer Kolothum
2021-07-29 14:59   ` kernel test robot
2021-07-29 10:40 ` [PATCH v3 4/4] KVM: arm64: Clear active_vmids on vCPU schedule out Shameer Kolothum
2021-08-03 11:40   ` Will Deacon
2021-08-03 12:55     ` Shameerali Kolothum Thodi
2021-08-03 15:30       ` Will Deacon [this message]
2021-08-03 15:56         ` Shameerali Kolothum Thodi
2021-08-06 12:24         ` Shameerali Kolothum Thodi
2021-08-09 13:09           ` Will Deacon
2021-08-09 13:48             ` Shameerali Kolothum Thodi
2021-08-11  8:47         ` Shameerali Kolothum Thodi
2021-10-11  6:06         ` Shameerali Kolothum Thodi

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