Re: [Qemu-devel] Would virtio support 64 bit address for vring virtqueue?

[Christian Borntraeger <borntraeger@de.ibm.com>]

On 28/08/13 13:45, Laszlo Ersek wrote:
> On 08/28/13 11:22, Xie, Huawei wrote:
>> Hi Stefan:
>> I think you mention the descriptor address? I mean the vring PFN register.
>> /* A 32-bit r/w PFN for the currently selected queue */
>> #define VIRTIO_PCI_QUEUE_PFN		8
>>
>> And the linux driver sample code:
>> iowrite32(virt_to_phys(info->queue) >> VIRTIO_PCI_QUEUE_ADDR_SHIFT,
>> 		  vp_dev->ioaddr + VIRTIO_PCI_QUEUE_PFN);
>>
>> -----Original Message-----
>> From: Stefan Hajnoczi [mailto:stefanha@gmail.com] 
>> Sent: Wednesday, August 28, 2013 4:07 PM
>> To: Xie, Huawei
>> Cc: qemu-devel@nongnu.org; rusty@rustcorp.com.au; Stefan Hajnoczi
>> Subject: Re: [Qemu-devel] Would virtio support 64 bit address for vring virtqueue?
>>
>> On Wed, Aug 28, 2013 at 03:18:39AM +0000, Xie, Huawei wrote:
>>> I am developing virtio user space poll mode network driver. We allocate vring physical memory from huge page. On VMs with less than 4GB  memory, it works well.  But on VMs with like 8GB memory, huge page are all allocated from high end memory.
>>> So would virtio support 64bit address for vring virtqueue?
>>
>> The vring takes guest physical addresses and the C type is __u64 (see /usr/include/linux/virtio_ring.h).  64-bit addresses are fine.
> 
> If you mean Queue Address field in the Virtio Header (section 2.2.2 in
> the virtio-0.9.5 specification), then please see 2.3 Virtqueue
> Configuration, step 3. You have to divide the guest-phys address by 4096
> and store the quotient. The Queue Address field takes a page frame
> number, not a page frame address.
> 
> This allows it to address up to 2^32 * 4096 == 16T bytes of RAM.

To be even more precise. This is only true for virtio-pci. virtio-ccw on s390x can define the
queue address as 64bit.

Christian