Re: [PATCH v5 00/15] vDPA shadow virtqueue

[Michael S. Tsirkin]

On Wed, Mar 09, 2022 at 11:38:35AM +0800, Jason Wang wrote:
> 
> 在 2022/3/8 下午8:16, Michael S. Tsirkin 写道:
> > On Tue, Mar 08, 2022 at 12:37:33PM +0100, Eugenio Perez Martin wrote:
> > > On Tue, Mar 8, 2022 at 11:48 AM Michael S. Tsirkin <mst@redhat.com> wrote:
> > > > On Tue, Mar 08, 2022 at 04:20:53PM +0800, Jason Wang wrote:
> > > > > > Not by itself but I'm not sure we can guarantee guest will not
> > > > > > attempt to use the IOVA addresses we are reserving down
> > > > > > the road.
> > > > > The IOVA is allocated via the listeners and stored in the iova tree
> > > > > per GPA range as IOVA->(GPA)->HVA.Guests will only see GPA, Qemu
> > > > > virtio core see GPA to HVA mapping. And we do a reverse lookup to find
> > > > > the HVA->IOVA we allocated previously.  So we have double check here:
> > > > > 
> > > > > 1) Qemu memory core to make sure the GPA that guest uses is valid
> > > > > 2) the IOVA tree that guarantees there will be no HVA beyond what
> > > > > guest can see is used
> > > > > 
> > > > > So technically, there's no way for the guest to use the IOVA address
> > > > > allocated for the shadow virtqueue.
> > > > > 
> > > > > Thanks
> > > > I mean, IOVA is programmed in the host hardware to translate to HPA, 
> > > > right?
> > > > 
> > > Yes, that's right if the device uses physical maps. Also to note, SVQ
> > > vring is allocated in multiples of host huge pages to avoid garbage or
> > > unintended access from the device.
> > > 
> > > If a vdpa device uses physical addresses, kernel vdpa will pin qemu
> > > memory first and then will send IOVA to HPA translation to hardware.
> > > But this IOVA space is not controlled by the guest, but by SVQ. If a
> > > guest's virtqueue buffer cannot be translated first to GPA, it will
> > > not be forwarded.
> > > 
> > > Thanks!
> > Right. So if guests send a buffer where buffer address overlaps the
> > range we used for the SVQ, then I think at the moment guest won't work.
> 
> 
> There's no way for a guest to do this, it can only use GPA

With a vIOMMU it can.

> but the Qemu
> won't let vDPA to use GPA as IOVA. Dedicated IOVA ranges were allocated for
> those GPA ranges so SVQ won't use IOVA that is overlapped with what Guest
> use.
> 
> Thanks
> 
> 
> >