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

[Anthony Liguori]

Yes, it was originally designed with the 16TB limit in mind.

PCI doesn't support 64-bit PIO operations so it would have required a
high/low register and additional magic.

When/if we switch to an MMIO region, we can switch to using a proper
64-bit register for setting up queues.

Regards,

Anthony Liguori



On Wed, Aug 28, 2013 at 10:30 AM, Xie, Huawei <address@hidden> wrote:
> I know it is PFN, that is why I want to use the high 12bit for combined write 
> in another reply, to fully address 64 bit address.
> Is the 16TB address space what it is originally designed for? If it is, that 
> will basically solve the problem. I will try later.
> Thanks for reminder
>
> -----Original Message-----
> From: Laszlo Ersek [mailto:address@hidden
> Sent: Wednesday, August 28, 2013 7:46 PM
> To: Xie, Huawei
> Cc: Stefan Hajnoczi; address@hidden; address@hidden; Stefan Hajnoczi
> Subject: Re: [Qemu-devel] Would virtio support 64 bit address for vring 
> virtqueue?
>
> 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.
>
> Laszlo
>
>