Re: [Xen-devel] Altp2m use with PML can deadlock Xen

[Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>]

On 5/10/19 5:42 PM, Tamas K Lengyel wrote:
> On Thu, May 9, 2019 at 10:19 AM Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:
> > On 09/05/2019 14:38, Tamas K Lengyel wrote:
> > > Hi all,
> > > I'm investigating an issue with altp2m that can easily be reproduced
> > > and leads to a hypervisor deadlock when PML is available in hardware.
> > > I haven't been able to trace down where the actual deadlock occurs.
> > >
> > > The problem seem to stem from hvm/vmx/vmcs.c:vmx_vcpu_flush_pml_buffer
> > > that calls p2m_change_type_one on all gfns that were recorded the PML
> > > buffer. The problem occurs when the PML buffer full vmexit happens
> > > while the active p2m is an altp2m. Switching  p2m_change_type_one to
> > > work with the altp2m instead of the hostp2m however results in EPT
> > > misconfiguration crashes.
> > >
> > > Adding to the issue is that it seem to only occur when the altp2m has
> > > remapped GFNs. Since PML records entries based on GFN leads me to
> > > question whether it is safe at all to use PML when altp2m is used with
> > > GFN remapping. However, AFAICT the GFNs in the PML buffer are not the
> > > remapped GFNs and my understanding is that it should be safe as long
> > > as the GFNs being tracked by PML are never the remapped GFNs.
> > >
> > > Booting Xen with ept=pml=0 resolves the issue.
> > >
> > > If anyone has any insight into what might be happening, please let me know.
> >
> > I could have sworn that George spotted a problem here and fixed it.  I
> > shouldn't be surprised if we have more.
> >
> > The problem that PML introduced (and this is mostly my fault, as I
> > suggested the buggy solution) is that the vmexit handler from one vcpu
> > pauses others to drain the PML queue into the dirty bitmap.  Overall I
> > wasn't happy with the design and I've got some ideas to improve it, but
> > within the scope of how altp2m was engineered, I proposed
> > domain_pause_except_self().
> >
> > As it turns out, that is vulnerable to deadlocks when you get two vcpus
> > trying to pause each other and waiting for each other to become
> > de-scheduled.
> Makes sense.
>
> > I see this has been reused by the altp2m code, but it *should* be safe
> > to deadlocks now that it takes the hypercall_deadlock_mutext.
> Is that already in staging or your x86-next branch? I would like to
> verify that the problem is still present or not with that change. I
> tested with Xen 4.12 release and that definitely still deadlocks.
I don't know if Andrew is talking about this patch (probably not, but it 
looks at least related):
http://xenbits.xen.org/gitweb/?p=xen.git;a=commitdiff;h=24d5282527f4647907b3572820b5335c15cd0356;hp=29d28b29190ba09d53ae7e475108def84e16e363

Since there's a "Release-acked" tag on it, I think it's in 4.12.


Thanks,
Razvan

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel