Re: [Qemu-devel] [PATCH V3 10/11] vcpu: introduce lockmap

[Avi Kivity <avi@redhat.com>]

On 09/11/2012 03:25 PM, Jan Kiszka wrote:
>>>>
>>>> Most DMA today happens without the big qemu lock.  We only need to
>>>> convert the paths that actually access memory, these are the block and
>>>> network layers (for the respective devices).
>>>
>>> ...and the devices themselves, of course.
>> 
>> Right, for descriptors and stuff.  So a guest can set a DMA address to
>> point at its own BAR, and cause qemu to deadlock.
>> 
>> The problem is not limited to locking.  A guest can also cause a write
>> to a BAR to initiate a synchronous write with the same address and data,
>> triggering infinite recursion.
>> 
>> Perhaps one fix is the mythical DMA API, which will provide each DMA
>> initiator with its own view of memory (a private MemoryRegion root
>> node).  Even that can be worked around with a pair of devices accessing
>> each other.
> 
> Hmm, don't see an alternative to runtime loop detection yet.

See an expensive one on the other branch of this thread.

> 
>> 
>>>
>>>>
>>>>> The other option is to keep DMA requests issued by devices synchronous
>>>>> but let them fail if we are about to lock up. Still requires changes,
>>>>> but is probably more comprehensible for device model developers.
>>>>
>>>> How do you handle failures?
>>>
>>> By not sending a network frame or dropping an incoming one, e.g., and
>>> signaling this in a device specific way.
>> 
>> Doesn't work for block devices.
> 
> Because the block layer API cannot report errors to the devices? What
> happens if there is a real I/O error?

We report real I/O errors.  But if we report a transient error due to
some lock being taken as an I/O error, the guest will take unwarranted
action.

If the errors are not expected in normal operation (we can avoid them if
all DMA is to real RAM) then this is an acceptable solution.  Still it
generates a lot of rarely used code paths and so isn't very good for
security.

-- 
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