Re: [Qemu-devel] [PATCH] vhost-user spec: Clarify policy on setting log_base

[Michael S. Tsirkin]

On Tue, Nov 28, 2017 at 04:04:21PM +0000, Dr. David Alan Gilbert wrote:
> * Victor Kaplansky (address@hidden) wrote:
> > From: Victor Kaplansky <address@hidden>
> > 
> > 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 <address@hidden>
> > Suggested-by: Maxime Coquelin <address@hidden>
> > ---
> >  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 / address@hidden / Manchester, UK