[Qemu-devel] Re: [PATCH v5 4/5] Inter-VM shared memory PCI device

[Cam Macdonell]

On Wed, May 12, 2010 at 9:49 AM, Avi Kivity <address@hidden> wrote:
> On 05/10/2010 07:48 PM, Cam Macdonell wrote:
>>
>> On Mon, May 10, 2010 at 10:40 AM, Avi Kivity<address@hidden>  wrote:
>>
>>>
>>> On 05/10/2010 06:41 PM, Cam Macdonell wrote:
>>>
>>>>
>>>>
>>>>>
>>>>> What would happen to any data written to the BAR before the the
>>>>> handshake
>>>>> completed?  I think it would disappear.
>>>>>
>>>>>
>>>>
>>>> But, the BAR isn't there until the handshake is completed.  Only after
>>>> receiving the shared memory fd does my device call pci_register_bar()
>>>> in the callback function.  So there may be a case with BAR2 (the
>>>> shared memory BAR) missing during initialization.  FWIW, I haven't
>>>> encountered this.
>>>>
>>>>
>>>
>>> Well, that violates PCI.  You can't have a PCI device with no BAR, then
>>> have
>>> a BAR appear.  It may work since the BAR is registered a lot faster than
>>> the
>>> BIOS is able to peek at it, but it's a race nevertheless.
>>>
>>
>> Agreed.  I'll get Anthony's idea up and running.  It seems that is the
>> way forward.
>>
>
> What, with the separate allocation and memcpy?  Or another one?

Mapping in the memory when it is received from the server.

>
> Why can't we complete initialization before exposing the card and BAR?
>  Seems to be the simplest solution.

Looking at it more closely, you're right, the fds for shared
memory/eventfds are received in a fraction of a second, so that's why
I haven't seen any problems since the memory is mapped before the BIOS
detects and configures the device.

We can't block on a qemu char device (in anyway I can see) so we have
to handle mapping the memory BAR in the callback function.  But, we
can make the semantics that the VM ID is not set until the memory is
mapped.  So if the VM ID is -1, then the memory has not been mapped
yet, reads/writes work but don't do anything useful.  So the user can
detect the mapping of the memory
and it does not invalidate PCI since the BAR is always present, but
just not mapped to the shared memory.

Cam