Re: [Qemu-devel] [PATCH v4] tests/vhost-user-bridge: implement logging of dirty pages

[Victor Kaplansky <victork@redhat.com>]

On Tue, Nov 17, 2015 at 01:04:33PM +0200, Michael S. Tsirkin wrote:
> On Tue, Nov 17, 2015 at 12:04:06PM +0200, Victor Kaplansky wrote:
> > During migration devices continue writing to the guest's memory.
> > The writes has to be reported to QEMU. This change implements
> > minimal support in vhost-user-bridge required for successful
> > migration of a guest with virtio-net device.
> > 
> > Signed-off-by: Victor Kaplansky <victork@redhat.com>
> > ---
> > v4:
> >     - implement set_vring_enable()
> >     - stop the device on vubr_get_vring_base_exec() and
> >       start on setting kick_fd as a work-around to QEMU bug
> >       enabling vrings too early.
> 
> You do want to address that FIXME but at least it's
> a documented bug.
> 
> > v3:
> >     - Get rid of vhost_log_chunk_t. Just use uint8_t.
> >     - Implement vubr_set_log_fd_exec().
> >     - Kick the log if log_call_fd has been set up.
> >     - Mark bits in log table atomically to enable more then one
> >       simultaneous vhost-user backend.
> >     - Fix the calculations of required log table size in an
> >       assert.
> >     - Fix the coding style: only single space before assignment
> >       operator.
> >     - Add a comment on the hack to determine that queues are
> >       ready for processing.
> >     - Other minor cosmetic fixes.
> > v2:
> >     - use log_guest_addr for used ring reported by qemu instead of
> >       translating.
> >     - use mmap_size and mmap_offset defined in new
> >       VHOST_USER_SET_LOG_BASE interface. See the patch
> >       "vhost-user: modify SET_LOG_BASE to pass mmap size and
> >       offset".
> >     - start logging dirty pages only after the appropriate feature
> >       is set by a VHOST_USER_GET_PROTOCOL_FEATURES request.
> >     - updated TODO list.
> > 
> >  tests/vhost-user-bridge.c | 220 ++++++++++++++++++++++++++++++++++++++++------
> >  1 file changed, 195 insertions(+), 25 deletions(-)
> > 
> > diff --git a/tests/vhost-user-bridge.c b/tests/vhost-user-bridge.c
> > index 864f69e..7bdfc98 100644
> > --- a/tests/vhost-user-bridge.c
> > +++ b/tests/vhost-user-bridge.c
> > @@ -466,11 +507,39 @@ vubr_virtqueue_kick(VubrVirtq *vq)
> >  }
> >  
> >  static void
> > +vubr_log_page(uint8_t *log_table, uint64_t page)
> > +{
> > +    DPRINT("Logged dirty guest page: %"PRId64"\n", page);
> > +    atomic_or(&log_table[page / 8], 1 << (page % 8));
> 
> One thing you definitely want here is a check
> against log size.
> Otherwise it's a security hole:
> remote can corrupt your memory by making you
> set random bits outside the mapped buffer.
> 
> What to do on error? Probably abort.
> 

vubr_log_write() has an assert checking the address.
In the next version I'll replace if by an if statement.

> > +}
> > +
> > +static void
> > +vubr_log_write(VubrDev *dev, uint64_t address, uint64_t length)
> > +{
> > +    uint64_t page;
> > +
> > +    if (!(dev->features & (1ULL << VHOST_F_LOG_ALL)) ||
> > +        !dev->log_table || !length) {
> > +        return;
> > +    }
> > +
> > +    assert(dev->log_size > ((address + length - 1) / VHOST_LOG_PAGE / 8));
> > +
> > +    page = address / VHOST_LOG_PAGE;
> > +    while (page * VHOST_LOG_PAGE < address + length) {
> > +        vubr_log_page(dev->log_table, page);
> > +        page += VHOST_LOG_PAGE;
> > +    }
> > +    vubr_log_kick(dev);
> > +}
> > +

-- Victor