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

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

On Mon, May 18, 2009 at 03:33:20PM +0300, Avi Kivity wrote:
> Michael S. Tsirkin wrote:
>>>
>>>> On the other hand, in MSI-X mask bit is mandatory, not optional
>>>> so we'll have to support it for assigned devices at some point.
>>>>
>>>> If we are worried about speed of masking/unmasking MSI-X interrupts for
>>>> assigned devices (older kernels used to mask them, recent kernels leave
>>>> this to drivers) we will probably need to have MSI-X support in the
>>>> kernel, and have kernel examine the mask bit before injecting the
>>>> interrupt, just like real devices do.
>>>>         
>>> Yes.
>>>     
>>
>> Actually, if we do that, we'll need to address a race where a driver
>> has updated the mask bit in the window after we tested it
>> and before we inject the interrupt. Not sure how to do this.
>>   
>
> The driver can't tell if the interrupt came first, so it's a valid race  
> (real hardware has the same race).

The driver for real device can do a read followed by sync_irq to flush
out interrupts.

-- 
MST