From: Vitaly Kuznetsov <vkuznets@redhat.com>
To: Maxim Levitsky <mlevitsk@redhat.com>, kvm@vger.kernel.org
Cc: "open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)"
<linux-kernel@vger.kernel.org>, Jim Mattson <jmattson@google.com>,
Joerg Roedel <joro@8bytes.org>, Borislav Petkov <bp@alien8.de>,
Wanpeng Li <wanpengli@tencent.com>,
Paolo Bonzini <pbonzini@redhat.com>,
Thomas Gleixner <tglx@linutronix.de>,
"H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)"
<x86@kernel.org>, Sean Christopherson <seanjc@google.com>
Subject: Re: [PATCH v2 8/8] KVM: x86: hyper-v: Deactivate APICv only when AutoEOI feature is in use
Date: Mon, 19 Jul 2021 11:23:36 +0200 [thread overview]
Message-ID: <87tukqzmg7.fsf@vitty.brq.redhat.com> (raw)
In-Reply-To: <b2978f4868437db23208718b3850b8fb6c0409eb.camel@redhat.com>
Maxim Levitsky <mlevitsk@redhat.com> writes:
> On Mon, 2021-07-19 at 09:47 +0200, Vitaly Kuznetsov wrote:
>> Maxim Levitsky <mlevitsk@redhat.com> writes:
>>
>> > On Tue, 2021-07-13 at 17:20 +0300, Maxim Levitsky wrote:
>> > > From: Vitaly Kuznetsov <vkuznets@redhat.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@redhat.com>
>> > > Signed-off-by: Maxim Levitsky <mlevitsk@redhat.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
next prev parent reply other threads:[~2021-07-19 9:23 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-07-13 14:20 [PATCH v2 0/8] My AVIC patch queue Maxim Levitsky
2021-07-13 14:20 ` [PATCH v2 1/8] KVM: SVM: svm_set_vintr don't warn if AVIC is active but is about to be deactivated Maxim Levitsky
2021-07-13 14:20 ` [PATCH v2 2/8] KVM: SVM: tweak warning about enabled AVIC on nested entry Maxim Levitsky
2021-07-13 14:20 ` [PATCH v2 3/8] KVM: SVM: use vmcb01 in svm_refresh_apicv_exec_ctrl Maxim Levitsky
2021-07-13 14:20 ` [PATCH v2 4/8] KVM: x86: APICv: drop immediate APICv disablement on current vCPU Maxim Levitsky
2021-07-13 14:20 ` [PATCH v2 5/8] KVM: x86: APICv: fix race in kvm_request_apicv_update on SVM Maxim Levitsky
2021-07-26 22:34 ` Paolo Bonzini
2021-07-27 13:22 ` Maxim Levitsky
2021-07-13 14:20 ` [PATCH v2 6/8] KVM: SVM: add warning for mistmatch between AVIC state and AVIC access page state Maxim Levitsky
2021-07-13 14:20 ` [PATCH v2 7/8] KVM: SVM: call avic_vcpu_load/avic_vcpu_put when enabling/disabling AVIC Maxim Levitsky
2021-07-13 14:20 ` [PATCH v2 8/8] KVM: x86: hyper-v: Deactivate APICv only when AutoEOI feature is in use Maxim Levitsky
2021-07-18 12:13 ` Maxim Levitsky
2021-07-19 7:47 ` Vitaly Kuznetsov
2021-07-19 9:00 ` Maxim Levitsky
2021-07-19 9:23 ` Vitaly Kuznetsov [this message]
2021-07-19 9:58 ` Maxim Levitsky
2021-07-19 18:49 ` Sean Christopherson
2021-07-20 9:40 ` Maxim Levitsky
2021-07-22 9:12 ` KVM's support for non default APIC base Maxim Levitsky
2021-08-02 9:20 ` Maxim Levitsky
2021-08-06 21:55 ` Sean Christopherson
2021-08-09 9:40 ` Maxim Levitsky
2021-08-09 15:57 ` Sean Christopherson
2021-08-09 16:47 ` Jim Mattson
2021-08-10 20:42 ` Maxim Levitsky
2021-07-22 17:35 ` [PATCH v2 8/8] KVM: x86: hyper-v: Deactivate APICv only when AutoEOI feature is in use Maxim Levitsky
2021-07-22 19:06 ` Sean Christopherson
2021-07-27 13:05 ` Maxim Levitsky
2021-07-27 17:48 ` Ben Gardon
2021-07-27 18:17 ` Sean Christopherson
2021-07-29 14:10 ` Maxim Levitsky
2021-07-26 17:24 ` [PATCH v2 0/8] My AVIC patch queue Paolo Bonzini
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