linux-kernel - Re: [PATCH v2 8/8] KVM: x86: hyper-v: Deactivate APICv only when AutoEOI feature is in use

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Message-ID: <87tukqzmg7.fsf@vitty.brq.redhat.com>
Date:   Mon, 19 Jul 2021 11:23:36 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>, kvm@...r.kernel.org
Cc:     "open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" 
        <linux-kernel@...r.kernel.org>, Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, Borislav Petkov <bp@...en8.de>,
        Wanpeng Li <wanpengli@...cent.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Sean Christopherson <seanjc@...gle.com>
Subject: Re: [PATCH v2 8/8] KVM: x86: hyper-v: Deactivate APICv only when
 AutoEOI feature is in use

Maxim Levitsky <mlevitsk@...hat.com> writes:

> On Mon, 2021-07-19 at 09:47 +0200, Vitaly Kuznetsov wrote:
>> Maxim Levitsky <mlevitsk@...hat.com> writes:
>> 
>> > On Tue, 2021-07-13 at 17:20 +0300, Maxim Levitsky wrote:
>> > > From: Vitaly Kuznetsov <vkuznets@...hat.com>
>> > > 
>> > > APICV_INHIBIT_REASON_HYPERV is currently unconditionally forced upon
>> > > SynIC activation as SynIC's AutoEOI is incompatible with APICv/AVIC. It is,
>> > > however, possible to track whether the feature was actually used by the
>> > > guest and only inhibit APICv/AVIC when needed.
>> > > 
>> > > TLFS suggests a dedicated 'HV_DEPRECATING_AEOI_RECOMMENDED' flag to let
>> > > Windows know that AutoEOI feature should be avoided. While it's up to
>> > > KVM userspace to set the flag, KVM can help a bit by exposing global
>> > > APICv/AVIC enablement: in case APICv/AVIC usage is impossible, AutoEOI
>> > > is still preferred.
>> > > Maxim:
>> > >    - added SRCU lock drop around call to kvm_request_apicv_update
>> > >    - always set HV_DEPRECATING_AEOI_RECOMMENDED in kvm_get_hv_cpuid,
>> > >      since this feature can be used regardless of AVIC
>> > > 
>> > > Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
>> > > Signed-off-by: Maxim Levitsky <mlevitsk@...hat.com>
>> > > ---
>> > >  arch/x86/include/asm/kvm_host.h |  3 +++
>> > >  arch/x86/kvm/hyperv.c           | 34 +++++++++++++++++++++++++++------
>> > >  2 files changed, 31 insertions(+), 6 deletions(-)
>> > > 
>> > > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
>> > > index e11d64aa0bcd..f900dca58af8 100644
>> > > --- a/arch/x86/include/asm/kvm_host.h
>> > > +++ b/arch/x86/include/asm/kvm_host.h
>> > > @@ -956,6 +956,9 @@ struct kvm_hv {
>> > >  	/* How many vCPUs have VP index != vCPU index */
>> > >  	atomic_t num_mismatched_vp_indexes;
>> > >  
>> > > +	/* How many SynICs use 'AutoEOI' feature */
>> > > +	atomic_t synic_auto_eoi_used;
>> > > +
>> > >  	struct hv_partition_assist_pg *hv_pa_pg;
>> > >  	struct kvm_hv_syndbg hv_syndbg;
>> > >  };
>> > > diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
>> > > index b07592ca92f0..6bf47a583d0e 100644
>> > > --- a/arch/x86/kvm/hyperv.c
>> > > +++ b/arch/x86/kvm/hyperv.c
>> > > @@ -85,9 +85,22 @@ static bool synic_has_vector_auto_eoi(struct kvm_vcpu_hv_synic *synic,
>> > >  	return false;
>> > >  }
>> > >  
>> > > +
>> > > +static void synic_toggle_avic(struct kvm_vcpu *vcpu, bool activate)
>> > > +{
>> > > +	srcu_read_unlock(&vcpu->kvm->srcu, vcpu->srcu_idx);
>> > > +	kvm_request_apicv_update(vcpu->kvm, activate,
>> > > +			APICV_INHIBIT_REASON_HYPERV);
>> > > +	vcpu->srcu_idx = srcu_read_lock(&vcpu->kvm->srcu);
>> > > +}
>> > 
>> > Well turns out that this patch still doesn't work (on this
>> > weekend I found out that all my AVIC enabled VMs hang on reboot).
>> > 
>> > I finally found out what prompted me back then to make srcu lock drop
>> > in synic_update_vector conditional on whether the write was done
>> > by the host.
>> >  
>> > Turns out that while KVM_SET_MSRS does take the kvm->srcu lock,
>> > it stores the returned srcu index in a local variable and not
>> > in vcpu->srcu_idx, thus the lock drop in synic_toggle_avic
>> > doesn't work.
>> >  
>> > So it is likely that I have seen it not work, and blamed 
>> > KVM_SET_MSRS for not taking the srcu lock which was a wrong assumption.
>> >  
>> > I am more inclined to fix this by just tracking if we hold the srcu
>> > lock on each VCPU manually, just as we track the srcu index anyway,
>> > and then kvm_request_apicv_update can use this to drop the srcu
>> > lock when needed.
>> > 
>> 
>> Would it be possible to use some magic value in 'vcpu->srcu_idx' and not
>> introduce a new 'srcu_ls_locked' flag?
>
> Well, currently the returned index value from srcu_read_lock is opaque 
> (and we have two SRCU implementations and both I think return small positive numbers, 
> but I haven't studied them in depth).
>  
> We can ask the people that maintain SRCU to reserve a number (like -1)
> or so.
> I probably first add the 'srcu_is_locked' thought and then as a follow up patch
> remove it if they agree.
>

Ah, OK. BTW, I've just discovered srcu_read_lock_held() which sounds
like the function we need but unfortunately it is not.

-- 
Vitaly