On Tue, Nov 28, 2017 at 03:46:44PM +0200, Victor Kaplansky 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. > > Signed-off-by: Victor Kaplansky <vkapl...@redhat.com> > Suggested-by: Maxime Coquelin <maxime.coque...@redhat.com>
I'm not sure we can do this. Log resizing as a result of memory hotplug might force log base changes. Backends need to use something like rcu to avoid need for locking. Apropos I wonder whether it's a bug that vhost_dev_start calls vhost_set_log_base after starting rings. Same question for the iotlb callback. > --- > 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
