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>
> ---
> 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.
> +
Do we actually issue VHOST_USER_SET_LOG_BASE when ring is started?
Can we put it simpler:
master must not send VHOST_USER_SET_LOG_BASE when ring
is started and logging of used ring writes is started.
?
> 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
