Re: [Qemu-devel] [PATCH 6/6] spec/vhost-user spec: Add IOMMU support

[Maxime Coquelin <maxime.coquelin@redhat.com>]

On 05/17/2017 04:53 AM, Jason Wang wrote:
> 
> 
> On 2017年05月16日 23:16, Michael S. Tsirkin wrote:
>> On Mon, May 15, 2017 at 01:45:28PM +0800, Jason Wang wrote:
>>>
>>> On 2017年05月13日 08:02, Michael S. Tsirkin wrote:
>>>> On Fri, May 12, 2017 at 04:21:58PM +0200, Maxime Coquelin wrote:
>>>>> On 05/11/2017 08:25 PM, Michael S. Tsirkin wrote:
>>>>>> On Thu, May 11, 2017 at 02:32:46PM +0200, Maxime Coquelin wrote:
>>>>>>> This patch specifies and implements the master/slave communication
>>>>>>> to support device IOTLB in slave.
>>>>>>>
>>>>>>> The vhost_iotlb_msg structure introduced for kernel backends is
>>>>>>> re-used, making the design close between the two backends.
>>>>>>>
>>>>>>> An exception is the use of the secondary channel to enable the
>>>>>>> slave to send IOTLB miss requests to the master.
>>>>>>>
>>>>>>> Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
>>>>>>> ---
>>>>>>>     docs/specs/vhost-user.txt | 75 
>>>>>>> +++++++++++++++++++++++++++++++++++++++++++++++
>>>>>>>     hw/virtio/vhost-user.c    | 31 ++++++++++++++++++++
>>>>>>>     2 files changed, 106 insertions(+)
>>>>>>>
>>>>>>> diff --git a/docs/specs/vhost-user.txt b/docs/specs/vhost-user.txt
>>>>>>> index 5fa7016..4a1f0c3 100644
>>>>>>> --- a/docs/specs/vhost-user.txt
>>>>>>> +++ b/docs/specs/vhost-user.txt
>>>>>>> @@ -97,6 +97,23 @@ Depending on the request type, payload can be:
>>>>>>>        log offset: offset from start of supplied file descriptor
>>>>>>>            where logging starts (i.e. where guest address 0 would 
>>>>>>> be logged)
>>>>>>> + * An IOTLB message
>>>>>>> +   ---------------------------------------------------------
>>>>>>> +   | iova | size | user address | permissions flags | type |
>>>>>>> +   ---------------------------------------------------------
>>>>>>> +
>>>>>>> +   IOVA: a 64-bit guest I/O virtual address
>>>>>> guest -> VM
>>>>> Ok.
>>>>>
>>>>>>> +   Size: a 64-bit size
>>>>>> How do you specify "all memory"? give special meaning to size 0?
>>>>> Good point, it does not support all memory currently.
>>>>> It is not vhost-user specific, but general to the vhost 
>>>>> implementation.
>>>> But iommu needs it to support passthrough.
>>> Probably not, we will just pass the mappings in vhost_memory_region to
>>> vhost. Its memory_size is also a __u64.
>>>
>>> Thanks
>> That's different since that's chunks of qemu virtual memory.
>>
>> IOMMU maps IOVA to GPA.
>>
> 
> But we're in fact cache IOVA -> HVA mapping in the remote IOTLB. When 
> passthrough mode is enabled, IOVA == GPA, so passing mappings in 
> vhost_memory_region should be fine.

Not sure this is a good idea, because when configured in passthrough,
QEMU will see the IOMMU as enabled, so the the VIRTIO_F_IOMMU_PLATFORM
feature will be negotiated if both guest and backend support it.
So how the backend will know whether it should directly pick the
translation directly into the vhost_memory_region, or translate it
through the device IOTLB?

Maybe the solution would be for QEMU to wrap "all memory" IOTLB updates
& invalidations to vhost_memory_regions, since the backend won't anyway
be able to perform accesses outside these regions?

> The only possible "issue" with "all memory" is if you can not use a 
> single TLB invalidation to invalidate all caches in remote TLB.

If needed, maybe we could introduce a new VHOST_IOTLB_INVALIDATE message
type?
For older kernel backend that doesn't support it, -EINVAL will be
returned, so QEMU could handle it another way in this case.

> But this 
> is only theoretical problem since it only happen when we have a 1 byte 
> mapping [2^64 - 1, 2^64) cached in remote TLB. Consider:
> 
> - E.g intel IOMMU has a range limitation for invalidation (1G currently)
> - Looks like all existed IOMMU use page aligned mappings
> 
> It was probably not a big issue. And for safety we could use two 
> invalidations to make sure all caches were flushed remotely. Or just 
> change the protocol from start, size to start, end. Vhost-kernel is 
> probably too late for this change, but I'm still not quite sure it is 
> worthwhile.

I'm not for diverging the protocol between kernel & user backends.

Thanks,
Maxime

> Thanks