* Victor Kaplansky (vkapl...@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
> 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.
> +
> 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 / dgilb...@redhat.com / Manchester, UK
