linux-kernel - Re: [PATCH v2 08/12] KVM: SVM: Update AVIC settings when changing APIC mode

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Date:   Wed, 04 May 2022 14:46:14 +0300
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     pbonzini@...hat.com, seanjc@...gle.com, joro@...tes.org,
        jon.grimm@....com, wei.huang2@....com, terry.bowman@....com
Subject: Re: [PATCH v2 08/12] KVM: SVM: Update AVIC settings when changing
 APIC mode

On Tue, 2022-05-03 at 20:04 +0700, Suravee Suthikulpanit wrote:
> Maxim,
> 
> On 5/3/22 12:13 AM, Maxim Levitsky wrote:
> > > In the kvm/queue branch, I found a regression on nested SVM guest, where L2 guest cannot
> > > launch. The bad commit is:
> > > 
> > > commit a4cfff3f0f8c07f1f7873a82bdeb3995807dac8c (bisect)
> > > Merge: 42dcbe7d8bac 8d5678a76689
> > > Author: Paolo Bonzini<pbonzini@...hat.com>
> > > Date:   Fri Apr 8 12:43:40 2022 -0400
> > > 
> > >       Merge branch 'kvm-older-features' into HEAD
> > > 
> > >       Merge branch for features that did not make it into 5.18:
> > > 
> > >       * New ioctls to get/set TSC frequency for a whole VM
> > > 
> > >       * Allow userspace to opt out of hypercall patching
> > > 
> > >       Nested virtualization improvements for AMD:
> > > 
> > >       * Support for "nested nested" optimizations (nested vVMLOAD/VMSAVE,
> > >         nested vGIF)
> > > 
> > >       * Allow AVIC to co-exist with a nested guest running
> > > 
> > >       * Fixes for LBR virtualizations when a nested guest is running,
> > >         and nested LBR virtualization support
> > > 
> > >       * PAUSE filtering for nested hypervisors
> > > 
> > >       Guest support:
> > > 
> > >       * Decoupling of vcpu_is_preempted from PV spinlocks
> > > 
> > >       Signed-off-by: Paolo Bonzini<pbonzini@...hat.com>
> > > 
> > > I am still working on the bisect into the merge commits.
> > > 
> > > Regards,
> > > Suravee
> > > 
> > What happens when the guest can't launch? It sure works for me for kvm/queue
> > from yesterday.
> > 
> > I'll test again tomorrow.
> 
> I have bisected it to this commit:
> 
> commit 74fd41ed16fd71725e69e2cb90b755505326c2e6
> Author: Maxim Levitsky <mlevitsk@...hat.com>
> Date:   Tue Mar 22 19:40:47 2022 +0200
> 
>      KVM: x86: nSVM: support PAUSE filtering when L0 doesn't intercept PAUSE
> 
>      Expose the pause filtering and threshold in the guest CPUID
>      and support PAUSE filtering when possible:
> 
>      - If the L0 doesn't intercept PAUSE (cpu_pm=on), then allow L1 to
>        have full control over PAUSE filtering.
> 
>      - if the L1 doesn't intercept PAUSE, use host values and update
>        the adaptive count/threshold even when running nested.
> 
>      - Otherwise always exit to L1; it is not really possible to merge
>        the fields correctly.  It is expected that in this case, userspace
>        will not enable this feature in the guest CPUID, to avoid having the
>        guest update both fields pointlessly.
> 
>      Signed-off-by: Maxim Levitsky <mlevitsk@...hat.com>
>      Message-Id: <20220322174050.241850-4-mlevitsk@...hat.com>
>      Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> 
> I can revert this one or specify pause_filter_count=0 pause_filter_thresh=0,
> and then I can boot the L2 guest.
> 
> Regards,
> Suravee
> 

This is really wierd.

Could you share the qemu command line for L1 and L2 guest, and as much as possible
info on what happens when you boot L2? I tested latest kvm/queue and I don't see
any issues with booting nested guest.

Which hardware you test on? I test on Zen2 (3970X) mostly.

How many vCPUs L2 has? Could you do a kvm trace of the L2, from L1,
to see what it does prior to hang?


Best regards,
	Maxim Levitsky