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

[Maxime Coquelin]

On 05/19/2017 08:48 AM, Jason Wang wrote:
> On 2017年05月17日 22:10, Maxime Coquelin wrote:
> > 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 <address@hidden>
> > > > > > > > > ---
> > > > > > > > >     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?
>
> This no need for backend to know about this, since IOVA equals to GPA, 
> vhost_memory_region stores IOVA -> HVA mapping. If we pass them, device 
> IOTLB should work as usual?

Ok, I think there were a misunderstanding. I understood you said there
were no need to use the device IOTLB in this case.

> > 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?
>
> This is just what I mean, you can refer Peter's series. >
> > > 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.
>
> We could, but not sure it was really needed.

I meant VHOST_IOTLB_INVALIDATE_ALL, and yes, I'm not sure this is
needed. But this is an option we have it turns out to be at some point.

Thanks,
Maxime