Re: 5.1.0-rc1 regression: reset fails with kvm and -cpu host

[Dr. David Alan Gilbert]

* Eduardo Habkost (ehabkost@redhat.com) wrote:
> On Wed, Jul 22, 2020 at 04:47:32PM -0400, Eduardo Habkost wrote:
> > On Wed, Jul 22, 2020 at 08:05:01PM +0200, Jan Kiszka wrote:
> > > On 22.07.20 19:35, Eduardo Habkost wrote:
> > > > Hi Jan,
> > > > 
> > > > What was the last version where it worked for you?  Does using
> > > > "-cpu host,-vmx" help?
> > > 
> > > Yeah, -vmx does indeed help.
> > > 
> > > I didn't have the time to bisect yet. Just check my reflog, picked
> > > eb6490f544, and that works.
> > 
> > Thanks!
> > 
> > I could reproduce it locally[1], I will bisect it.
> > 
> > The good news is that "-cpu host,+vmx" still works, on commit
> > eb6490f544.
> > 
> > [1] Linux 5.6.19-300.fc32.x86_64, Intel Core i7-8665U CPU.
> 
> Bisected to:
> 
> commit b16c0e20c74218f2d69710cedad11da7dd4d2190
> Author: Paolo Bonzini <pbonzini@redhat.com>
> Date:   Wed May 20 10:49:22 2020 -0400
> 
>     KVM: add support for AMD nested live migration
> 
>     Support for nested guest live migration is part of Linux 5.8, add the
>     corresponding code to QEMU.  The migration format consists of a few
>     flags, is an opaque 4k blob.
> 
>     The blob is in VMCB format (the control area represents the L1 VMCB
>     control fields, the save area represents the pre-vmentry state; KVM does
>     not use the host save area since the AMD manual allows that) but QEMU
>     does not really care about that.  However, the flags need to be
>     copied to hflags/hflags2 and back.
> 
>     In addition, support for retrieving and setting the AMD nested 
> virtualization
>     states allows the L1 guest to be reset while running a nested guest, but
>     a small bug in CPU reset needs to be fixed for that to work.
> 
>     Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Guesswork led me to try reverting the chunk in kvm_put_nested_state;
without it the reset seems to work; I can't explain that code though.

Dave

> 
> -- 
> Eduardo
> 
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK