On Tue, Nov 28, 2017 at 04:04:21PM +0000, Dr. David Alan Gilbert wrote:
> * Victor Kaplansky (vkapl...@redhat.com) wrote:
> > From: Victor Kaplansky <vkaplans@dell9020.localdomain>
> >
> > If we allow qemu to change logging area after it was already established,
> > it may require from the backend to acquire a lock on each access to
> > the log_base, which has a potential quite a big performance hit.
> >
> > Thus we would like to clarify in the spec, that qemu is not expected
> > to resize or remap the logging area, and backend implementations
> > can safely ignore subsequent requests to log_base modifications.
>
> There's quite a bit of code in vhost to cope with changes in mappings
> of regions; and there's already code in there to handle log size
> changes:
>
> static inline void vhost_dev_log_resize(struct vhost_dev *dev, uint64_t size)
> {
> struct vhost_log *log = vhost_log_get(size, vhost_dev_log_is_shared(dev));
> uint64_t log_base = (uintptr_t)log->log;
> int r;
>
> /* inform backend of log switching, this must be done before
> releasing the current log, to ensure no logging is lost */
> r = dev->vhost_ops->vhost_set_log_base(dev, log_base, log);
>
> so when is a log resize legal?
>
> Dave
Log base has nothing to do with log resize.
It supplies the physical address of the ring
for dirty logging purposes.
> > Signed-off-by: Victor Kaplansky <vkapl...@redhat.com>
> > Suggested-by: Maxime Coquelin <maxime.coque...@redhat.com>
> > ---
> > docs/interop/vhost-user.txt | 6 ++++++
> > 1 file changed, 6 insertions(+)
> >
> > diff --git a/docs/interop/vhost-user.txt b/docs/interop/vhost-user.txt
> > index 954771d0d8..7ab31e57ef 100644
> > --- a/docs/interop/vhost-user.txt
> > +++ b/docs/interop/vhost-user.txt
> > @@ -257,6 +257,12 @@ Where addr is the guest physical address.
> >
> > Use atomic operations, as the log may be concurrently manipulated.
> >
> > +Note that master is not expected to issue more than one
> > VHOST_USER_SET_LOG_BASE
> > +request before the rings are fully stopped by the master. Thus no
> > modifications
> > +to log_base address are allowed before the rings are restated and the
> > client
> > +can ignore all subsequent VHOST_USER_SET_LOG_BASE requests after the
> > log_base
> > +address has been established.
> > +
> > Note that when logging modifications to the used ring (when
> > VHOST_VRING_F_LOG
> > is set for this ring), log_guest_addr should be used to calculate the log
> > offset: the write to first byte of the used ring is logged at this offset
> > from
> > --
> > 2.14.2
> >
> --
> Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
