Re: [Qemu-devel] [PATCH] VirtIO Support for Memory Regions

[Anthony Liguori <anthony@codemonkey.ws>]

On 02/24/2010 05:14 AM, Avi Kivity wrote:
> On 02/24/2010 08:20 AM, Cam Macdonell wrote:
>>
>>> Virtio is really a DMA engine.  One of the nice things about it's 
>>> design is
>>> that you can do things like transparent bounce buffering if needed.  
>>> Adding
>>> a mechanism like this breaks this abstract and turns virtio into 
>>> something
>>> that's more than I think it should be.
>>>
>>> For guest shared memory, what makes sense to me is to make use of 
>>> uio_pci
>>> within a guest to map bars in userspace.  The device can have the 
>>> first bar
>>> map a small MMIO region that is tied to a ioeventfd with KVM.  With 
>>> just
>>> qemu, it can be tied to a normal eventfd.  You can use irqfd with 
>>> KVM to tie
>>> an eventfd to the PCI irq.  Likewise, you can use a normal eventfd to
>>> emulate that.  I'd suggest using a /dev/shm file to act as the 
>>> shared memory
>>> segment.  You'll need a little magic within qemu_ram_alloc() to use 
>>> this
>>> (maybe qemu_ram_alloc() should take a context parameter so this can be
>>> setup).
>> Part of my goal with this patch was to support systems without PCI.
>> My previous patch
>
> Which?  The only one I know of is s390 (and it would be a pity not to 
> have shared memory for that).

uio provides insulation against devices and busses.  When you use uio in 
userspace, you don't have to have direct knowledge of the fact that a 
device is or isn't a PCI device.

Regards,

Anthony Liguori