Re: [PATCH v4] kvm: Use a bitmap for tracking used GSIs

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, May 18, 2009 at 05:46:09PM +0300, Avi Kivity wrote:
> Michael S. Tsirkin wrote:
>>> If it generates the interrupt after masking it in the msi-x entry, 
>>> we'll  see it.  If it generates the interrupt before masking it, it 
>>> may or may  not receive the interrupt, even on real hardware.
>>>     
>>
>> Yes but in the later case, real hardware must re-send the pending
>> interrupt after it is unmasked (that's the spec). We would just lose it.
>>   
>
> That's a different matter.  We need to buffer the interrupt pending bit,  
> and a way for userspace to either query that buffer or have a  
> conditional injection (inject_if_pending).

Here's the race as I see it: we discussed the possibility
of making kernel and user share and actual memory page,
and using that for MSI-X tables.

	host kernel want to send msi x message
	host kernel test mask bit: unmasked
	guest sets mask bit
	guest does read to flash msi writes
	guest does sync irq and makes sure there are no
                           outstanging interrupts
	---> at this stage guest expects not to get interrupts
	guest starts editing msix entry

	host kernel never saw mask so it sends message to the old address
	       or even a corrupted address which the guest is in
               the middle of editing
	bad things happen

This race is not easy to solve, except by catching writes to msix table,
and syncronising them with interrupt delivery.

-- 
MST