Re: [Qemu-devel] [BUG] Guest kernel divide error in kvm_unlock_kick

[Paolo Bonzini]

Il 22/09/2014 21:08, Chris Webb ha scritto:
> > Do you by chance have CONFIG_DEBUG_RODATA set?  In that case, the fix is
> > simply not to set it.
> 
> Absolutely right: my host and guest kernels do have CONFIG_DEBUG_RODATA set!
> 
> Your patch to use alternatives for VMCALL vs VMMCALL definitely fixed the
> divide-by-zero crashes I saw.
> 
> Given that I can easily use either (or both) of these solutions, is it be
> more efficient to turn off CONFIG_DEBUG_RODATA in the guest kernel so kvm
> can fix up the instructions in-place, or is using alternatives for
> VMCALL/VMMCALL as implemented by your patch just as good?

I posted a patch to use alternatives if CONFIG_DEBUG_RODATA is enabled,
but the bug is in KVM that explicitly documents "you can use any of
VMCALL or VMMCALL".

I'll also see to fix KVM, but the patch is still useful because a) KVM
would not patch a read-only text segment, so there would be a small
performance benefit; b) you cannot control already deployed hypervisors.

However, since there is a workaround, I won't push it into 3.17 so late
in the cycle.  Also, there's a chance that it is NACKed since it touches
non-KVM files.

Paolo