On Mon, 2021-07-19 at 11:23 +0200, Vitaly Kuznetsov wrote: > Maxim Levitsky <mlevitsk@xxxxxxxxxx> writes: > > > On Mon, 2021-07-19 at 09:47 +0200, Vitaly Kuznetsov wrote: > > > Maxim Levitsky <mlevitsk@xxxxxxxxxx> writes: > > > > > > > On Tue, 2021-07-13 at 17:20 +0300, Maxim Levitsky wrote: > > > > > From: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> > > > > > > > > > > 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@xxxxxxxxxx> > > > > > Signed-off-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx> > > > > > --- > > > > > 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. Yea, exactly this. :-( Best regards, Maxim Levitsky >