[Qemu-devel] Re: [PATCH v3 0/2] Inter-VM shared memory PCI device

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

On Thu, Mar 25, 2010 at 3:10 PM, Avi Kivity <avi@redhat.com> wrote:
> On 03/25/2010 08:17 PM, Cam Macdonell wrote:
>>
>>>> I had a hunch it was probably considered.  That explains why irqfd
>>>> doesn't have a datamatch field.  I guess supporting multiple MSI
>>>> vectors with one doorbell per guest isn't possible if one 1 bit of
>>>> information can be communicated.
>>>>
>>>>
>>>
>>> Actually you can have one doorbell supporting multiple vectors and
>>> guests,
>>> simply divide the data value into two bit fields, one for the vector and
>>> one
>>> for the guest.  A single write gets both values into the host, which can
>>> then use datamatch to trigger the correct eventfd (which is wired to an
>>> irqfd in another guest).
>>>
>>
>> At 4-bits per guest, a single write is then limited to 8 guests (with
>> 32-bit registers), we could got to 64-bit.
>>
>
> I meant a unicast doorbell: 16 bits for guest ID, 16 bits for vector number.

Ah, yes.  Who knew "two bit registers" is an ambiguous term.  Do you
strongly prefer the one doorbell design?