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

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Victor Kaplansky (vkaplans@redhat.com) wrote:
> > From: "Michael S. Tsirkin" <mst@redhat.com>
> > To: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> > Cc: "Victor Kaplansky" <vkaplans@redhat.com>, qemu-devel@nongnu.org, "Maxime Coquelin" <maxime.coquelin@redhat.com>
> > Sent: Tuesday, November 28, 2017 6:12:44 PM
> > Subject: Re: [Qemu-devel] [PATCH] vhost-user spec: Clarify policy on setting log_base
> > 
> > On Tue, Nov 28, 2017 at 04:04:21PM +0000, Dr. David Alan Gilbert wrote:
> > > * Victor Kaplansky (vkaplans@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.
> > 
> 
> Right. Just for complete picture, it also supplies the address for
> dirty logging of where descriptors can point, in other words for logging
> whole guest's physical memory. It is why a single log_base is enough for
> whole device, and there is no need to specify it for every ring separately.
> 
> Having said that, there is an obscure possibility to place the ring in
> a memory different from guest's physical memory, but as far as I know there
> is no backend that supports this now.
> 
> The introduction of this clarification to the vhost-user spec, either with
> *must* or without it, will enable us to fix in DPDK several BZs open for qemu
> and related to migration failure of MQ vhost-user.

OK thanks for the clarification Michael, Victor!

Dave

--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK