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

[Christian Borntraeger]

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:address@hidden 
>> Sent: Wednesday, August 28, 2013 4:07 PM
>> To: Xie, Huawei
>> Cc: address@hidden; address@hidden; 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