Re: PROBLEM: Regression of MMU causing guest VM application errors

[Sean Christopherson <sean.j.christopherson@intel.com>]

On Wed, Oct 16, 2019 at 11:28:57AM -0600, Alex Williamson wrote:
> On Wed, 16 Oct 2019 00:49:51 -0400
> Derek Yerger <derek@djy.llc> wrote:
> 
> > In at least Linux 5.2.7 via Fedora, up to 5.2.18, guest OS applications 
> > repeatedly crash with segfaults. The problem does not occur on 5.1.16.
> > 
> > System is running Fedora 29 with kernel 5.2.18. Guest OS is Windows 10 with an 
> > AMD Radeon 540 GPU passthrough. When on 5.2.7 or 5.2.18, specific windows 
> > applications frequently and repeatedly crash, throwing exceptions in random 
> > libraries. Going back to 5.1.16, the issue does not occur.
> > 
> > The host system is unaffected by the regression.
> > 
> > Keywords: kvm mmu pci passthrough vfio vfio-pci amdgpu
> > 
> > Possibly related: Unmerged [PATCH] KVM: x86/MMU: Zap all when removing memslot 
> > if VM has assigned device
> 
> That was never merged because it was superseded by:
> 
> d012a06ab1d2 Revert "KVM: x86/mmu: Zap only the relevant pages when removing a memslot"
> 
> That revert also induced this commit:
> 
> 002c5f73c508 KVM: x86/mmu: Reintroduce fast invalidate/zap for flushing memslot
> 
> Both of these were merged to stable, showing up in 5.2.11 and 5.2.16
> respectively, so seeing these sorts of issues might be considered a
> known issue on 5.2.7, but not 5.2.18 afaik.  Do you have a specific
> test that reliably reproduces the issue?  Thanks,

Also, does the failure reproduce on on 5.2.1 - 5.2.6?  The memslot debacle
exists on all flavors of 5.2.x, if the errors showed up in 5.2.7 then they
are being caused by something else.