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Re: [Xen-devel] Altp2m use with PML can deadlock Xen
- To: Tamas K Lengyel <tamas.k.lengyel@xxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Thu, 9 May 2019 17:19:40 +0100
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- Delivery-date: Thu, 09 May 2019 16:19:46 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Openpgp: preference=signencrypt
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.
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.
Anyway - sorry for not being more help, but I bet the problem is going
to be somewhere around vcpu pausing.
~Andrew
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