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

[Victor Kaplansky]

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.

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