[Bug 203845] Can't run qemu/kvm on 5.0.0 kernel (NULL pointer dereference)

[bugzilla-daemon@bugzilla.kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=203845

Sean Christopherson (sean.j.christopherson@intel.com) changed:

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--- Comment #1 from Sean Christopherson (sean.j.christopherson@intel.com) ---
The EIP and code stream puts this at the following line in
mmu_alloc_direct_roots():

  vcpu->arch.mmu->pae_root[i] = root | PT_PRESENT_MASK;

The code in question is only encountered on a 32-bit KVM with two-dimensional
paging (TDP) is disabled, i.e. without AMD's Nested Page Tables, which fits
your setup (i686-pae on Athlon).  What I can't figure out is how pae_root would
be NULL in this scenario.  There is one flow that I think could theoretically
result in a NULL pae_root, but it would require using nested virtualization,
which doesn't seem to be the case here.

I tried to test my theory but running a 32-bit KVM without TDP just hangs for
me, i.e. it appears to be broken on Intel VMX at least as far back as v4.20.

What was the last kernel that did work on your system?  That might help narrow
down when things went awry.  In the meantime, I'll try to debug/bisect the
issue I'm seeing as time allows.

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