Re: [Qemu-devel] [Patch] Support translating Guest physical address to Host virtual address.

[Anthony Liguori]

On 02/07/2010 10:31 AM, Avi Kivity wrote:
> > Only insofar as you don't have to deal with getting at the VM fd.  
> > You can avoid the problem by having the kvm ioctl interface take a 
> > pid or something.
>
>
> That's a racy interface.

The mechanism itself is racy.  That said, pid's don't recycle very 
quickly so the chances of running into a practical issue is quite small.

> Well, we need to provide a reasonable alternative.

I think this is the sort of thing that really needs to be a utility that 
lives outside of qemu.  I'm absolutely in favor of exposing enough 
internals to let people do interesting things provided it's reasonably 
correct.

> One might be to use -mempath (which is hacky by itself, but so far we 
> have no alternative) and use an external tool on the memory object to 
> poison it.  An advantage is that you can use it independently of kvm.

It would help if the actual requirements were spelled out a bit more.  
What exactly needs validating?  Do we need to validate that a poisoning 
a host physical address results in a very particular guest page getting 
poisoned?

Is it not enough to just choose a random anonymous memory area within 
the qemu process, generate an MCE to that location, see whether qemu 
SIGBUS's.  If it doesn't, validate that an MCE has been received in the 
guest?

But FWIW, I think a set of per-VM directories in sysfs could be very 
useful for this sort of debugging.

Maybe we should consider having the equivalent of a QMP-for-debugging 
session.  This would be a special QMP session that we basically provided 
no compatibility or even sanity guarantees that was specifically there 
for debugging.  I would expect that it be disabled in any production 
build (even perhaps even by default in the general build).

Regards,

Anthony Liguori