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

[Anthony Liguori <anthony@codemonkey.ws>]

On 03/08/2010 07:16 AM, Avi Kivity wrote:
> On 03/08/2010 03:03 PM, Paul Brook wrote:
>>> On 03/08/2010 12:53 AM, Paul Brook wrote:
>>>>> Support an inter-vm shared memory device that maps a shared-memory
>>>>> object as a PCI device in the guest.  This patch also supports
>>>>> interrupts between guest by communicating over a unix domain socket.
>>>>> This patch applies to the qemu-kvm repository.
>>>> No. All new devices should be fully qdev based.
>>>>
>>>> I suspect you've also ignored a load of coherency issues, 
>>>> especially when
>>>> not using KVM. As soon as you have shared memory in more than one host
>>>> thread/process you have to worry about memory barriers.
>>> Shouldn't it be sufficient to require the guest to issue barriers (and
>>> to ensure tcg honours the barriers, if someone wants this with tcg)?.
>> In a cross environment that becomes extremely hairy.  For example the 
>> x86
>> architecture effectively has an implicit write barrier before every 
>> store, and
>> an implicit read barrier before every load.
>
> Ah yes.  For cross tcg environments you can map the memory using mmio 
> callbacks instead of directly, and issue the appropriate barriers there.

Not good enough unless you want to severely restrict the use of shared 
memory within the guest.

For instance, it's going to be useful to assume that you atomic 
instructions remain atomic.  Crossing architecture boundaries here makes 
these assumptions invalid.  A barrier is not enough.

Shared memory only makes sense when using KVM.  In fact, we should 
actively disable the shared memory device when not using KVM.

Regards,

Anthony Liguori