[Qemu-devel] Re: [PATCH] Inter-VM shared memory PCI device

[Cam Macdonell <cam@cs.ualberta.ca>]

On Tue, Mar 9, 2010 at 10:28 AM, Avi Kivity <avi@redhat.com> wrote:
> On 03/09/2010 05:27 PM, Cam Macdonell wrote:
>>
>>>
>>>>  Registers are used
>>>> for synchronization between guests sharing the same memory object when
>>>> interrupts are supported (this requires using the shared memory server).
>>>>
>>>>
>>>
>>> How does the driver detect whether interrupts are supported or not?
>>>
>>
>> At the moment, the VM ID is set to -1 if interrupts aren't supported,
>> but that may not be the clearest way to do things.  With UIO is there
>> a way to detect if the interrupt pin is on?
>>
>
> I suggest not designing the device to uio.  Make it a good guest-independent
> device, and if uio doesn't fit it, change it.
>
> Why not support interrupts unconditionally?  Is the device useful without
> interrupts?

Currently my patch works with or without the shared memory server.  If
you give the parameter

-ivshmem 256,foo

then this will create (if necessary) and map /dev/shm/foo as the
shared region without interrupt support.  Some users of shared memory
are using it this way.

Going forward we can require the shared memory server and always have
interrupts enabled.

>
>>>> The Doorbell register is 16-bits, but is treated as two 8-bit values.
>>>>  The
>>>> upper 8-bits are used for the destination VM ID.  The lower 8-bits are
>>>> the
>>>> value which will be written to the destination VM and what the guest
>>>> status
>>>> register will be set to when the interrupt is trigger is the destination
>>>> guest.
>>>>
>>>>
>>>
>>> What happens when two interrupts are sent back-to-back to the same guest?
>>>  Will the first status value be lost?
>>>
>>
>> Right now, it would be.  I believe that eventfd has a counting
>> semaphore option, that could prevent loss of status (but limits what
>> the status could be).
>>
>
> It only counts the number of interrupts (and kvm will coalesce them anyway).

Right.

>
>> My understanding of uio_pci interrupt handling
>> is fairly new, but we could have the uio driver store the interrupt
>> statuses to avoid losing them.
>>
>
> There's nowhere to store them if we use ioeventfd/irqfd.  I think it's both
> easier and more efficient to leave this to the application (to store into
> shared memory).

Agreed.

Cam