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

[Avi Kivity <avi@redhat.com>]

Michael S. Tsirkin wrote:
> 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.
>   

You're right of course.  In any case this is premature, we'll have to 
see if this happens with any frequency.

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