On Tue, Nov 28, 2017 at 01:34:19PM -0500, Victor Kaplansky wrote: > > From: "Michael S. Tsirkin" <m...@redhat.com> > > To: "Victor Kaplansky" <vkapl...@redhat.com> > > Cc: qemu-devel@nongnu.org, "Maxime Coquelin" <maxime.coque...@redhat.com>, > > "Jason Wang" <jasow...@redhat.com> > > Sent: Tuesday, November 28, 2017 8:06:52 PM > > Subject: Re: [PATCH] vhost-user spec: Clarify policy on setting log_base > > > > 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. > > I agree. Memory hotplug during live migration will cause a > bunch of problems not only to dirty page logging, but to a > regular backend function, because memory regions have to be > redefined before descriptors pointing to the new memory > arrive a backend. Which means, that in DPDK we must to > guard by some kind of synchronization mechanism all accesses > to guest2qemu memory translation. This will have an impact > on the backend performace even with light-weighted RCU. > > So, I propose to handle memory hot plug by stopping rings, > then changing memory regions and log_base, and then re-enabling > rings. > > What do you say?
Why not have backend do this internally when it gets the log_base message? > > > > > 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. > > In vhost-user backend before vhost_user_set_log_base is called for first > time, log_base is initialized to zero. Which causes backend (DPDK) to > skip logging. Thus theoretically setting log_base on an active ring, > may cause to skip single (very first) dirty logging. In the reality, > it has very small chances to happen, since sending the response from > the backend to qemu by a socket takes much longer then a time gap > between reading log_base variable and using it. I suspect it's worth fixing though. > > > > > 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 > >
