Re: [PATCH] vhost-user: pass GPA instead of UVA

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Feb 05, 2026 at 11:22:26AM +0300, Vladimir Sementsov-Ogievskiy wrote:
> On 05.02.26 11:13, Michael S. Tsirkin wrote:
> > On Wed, Feb 04, 2026 at 12:23:39PM +0300, Vladimir Sementsov-Ogievskiy wrote:
> > > Unlike the kernel, vhost-user backend knows nothing about QEMUs
> > > userspace addresses. We can pass GPA instead and nothing changes.
> > > 
> > > The benefit: open the doors for further implementation of local
> > > migration (live-update) with passing open vhost-releated FDs
> > > through UNIX domain socket. This way the connection with backend
> > > kept live and untouched.
> > > 
> > > Without this change, we'll have to communicate with backend to
> > > inform it about UVA addresses change, but better is simply use
> > > more stable GPA numbers, like it's done for VDPA.
> > > 
> > > Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
> > > ---
> > >   hw/virtio/vhost-user.c | 30 ++++++++++++++++++++++++++++++
> > >   1 file changed, 30 insertions(+)
> > > 
> > > diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c
> > > index 63fa9a1b4b..f379408c95 100644
> > > --- a/hw/virtio/vhost-user.c
> > > +++ b/hw/virtio/vhost-user.c
> > > @@ -386,6 +386,24 @@ static int vhost_user_write(struct vhost_dev *dev, VhostUserMsg *msg,
> > >       CharFrontend *chr = u->user->chr;
> > >       int ret, size = VHOST_USER_HDR_SIZE + msg->hdr.size;
> > > +    if (msg->hdr.request == VHOST_USER_REM_MEM_REG ||
> > > +        msg->hdr.request == VHOST_USER_ADD_MEM_REG) {
> > > +
> > > +        /*
> > > +         * In QEMU we pass guest physical addresses to vhost-user server
> > > +         * instead of QEMUs virtual address. Server have no option to
> > > +         * distinguesh,
> > 
> > what does this mean? "Servers don't really care"?
> 
> Yes. Still, we can't be sure, what servers care about. But I think, as
> they don't have any access to QEMUs virtual address space, they can't
> have any logic that breaks if we start to pass GPA instead of UVA
> (imagine, that "accidentally" GPA and UVA are the same for the case).

right, just write something understandable pls.

> > 
> > > but we get a benefit: guest physical address is
> > > +         * stable during migration, and we don't have to reset memory
> > > +         * regions.
> > 
> > curious if PA 0 is valid on some systems (as opposed to VA).
> 
> Hmm, you are right, that's visible thing which differs for UVA and GPA.
> Still, checking UVA to be non-zero on server side seems rather strange,
> as the only use for these UVA addresses in the server is to calculate
> offsets. The server is only interested in offsets inside given memory
> region, not absolute UVA values.
> 
> And we don't write in spec that UVA can't be zero, and we already use
> GPA for VDPA.

poking at some servers to make sure can't hurt.

> > 
> > 
> > > +         *
> > > +         * Look also at vhost_user_vq_get_addr(): like in VDPA, we pass
> > > +         * physical addresses instead of user.
> > > +         */
> > > +
> > > +        msg->payload.mem_reg.region.userspace_addr =
> > > +            msg->payload.mem_reg.region.guest_phys_addr;
> > > +    }
> > > +
> > >       /*
> > >        * Some devices, like virtio-scsi, are implemented as a single vhost_dev,
> > >        * while others, like virtio-net, contain multiple vhost_devs. For
> > > @@ -3021,6 +3039,17 @@ static int vhost_user_check_device_state(struct vhost_dev *dev, Error **errp)
> > >       return 0;
> > >   }
> > > +static int vhost_user_vq_get_addr(struct vhost_dev *dev,
> > > +                                  struct vhost_vring_addr *addr,
> > > +                                  struct vhost_virtqueue *vq)
> > > +{
> > > +    assert(dev->vhost_ops->backend_type == VHOST_BACKEND_TYPE_USER);
> > > +    addr->desc_user_addr = (uint64_t)(unsigned long)vq->desc_phys;
> > > +    addr->avail_user_addr = (uint64_t)(unsigned long)vq->avail_phys;
> > > +    addr->used_user_addr = (uint64_t)(unsigned long)vq->used_phys;
> > > +    return 0;
> > > +}
> > > +
> > >   const VhostOps user_ops = {
> > >           .backend_type = VHOST_BACKEND_TYPE_USER,
> > >           .vhost_backend_init = vhost_user_backend_init,
> > > @@ -3059,4 +3088,5 @@ const VhostOps user_ops = {
> > >           .vhost_supports_device_state = vhost_user_supports_device_state,
> > >           .vhost_set_device_state_fd = vhost_user_set_device_state_fd,
> > >           .vhost_check_device_state = vhost_user_check_device_state,
> > > +        .vhost_vq_get_addr = vhost_user_vq_get_addr,
> > >   };
> > > -- 
> > > 2.52.0
> > 
> 
> 
> -- 
> Best regards,
> Vladimir