Re: NMI vs #PF clash

[Avi Kivity <avi@redhat.com>]

On 05/22/2012 06:33 PM, Linus Torvalds wrote:
> On Tue, May 22, 2012 at 7:27 AM, Steven Rostedt <rostedt@goodmis.org> wrote:
>>
>> Is reading it fast? Then we could do a two reads and only write when
>> needed.
> 
> Even better: we could do nothing at all.
> 
> We could just say: let's make sure that any #PF case that can happen
> in #NMI can also be re-done with arbitrary 'error_code' and 'struct
> regs' contents.
> 
> At that point, what could happen is
>  - #PF
>   - NMI
>    - #PF
>     - read cr2 for NMI fault
>     - handle the NMI #PF
>     - return from #PF
>   - return from #NMI
>   - read cr2 for original #PF fault - but get the NMI cr2 again
>   - hande the #PF again (this should be a no-op now)
>   - return from #PF
>  - instruction restart causes new #PF
>   - now we do the original page fault
> 
> So one option is to just make sure that the few cases (just the
> vmalloc area?) that NMI can trigger are all ok to be re-done with
> other state.
> 
> I note that right now we have
> 
>         if (unlikely(fault_in_kernel_space(address))) {
>                 if (!(error_code & (PF_RSVD | PF_USER | PF_PROT))) {
>                         if (vmalloc_fault(address) >= 0)
>                                 return;
> 
> and that the error_code check means that the retried NMI #PF would not
> go through that. But maybe we don't even need that check?
> 
> That error_code thing seems to literally be the only thing that keeps
> us from just re-doing the vmalloc_fault() silently.
> 

do_page_fault() is not the only code that relies on cr2; vmx_vcpu_run()
is the other.  If an NMI happens there, and takes a #PF, then the guest
will run with a bad cr2.

(svm saves and restores cr2 in microcode, and also provides a way to
mask NMIs, so it isn't vulnerable to this issue).

-- 
error compiling committee.c: too many arguments to function