Re: [Qemu-devel] [PATCH RFC 6/6] vhost-user: document migration log

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Jul 23, 2015 at 03:36:43AM +0200, Marc-André Lureau wrote:
> Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>

for some reason I didn't get 5/6.

> ---
>  docs/specs/vhost-user.txt | 40 ++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 40 insertions(+)
> 
> diff --git a/docs/specs/vhost-user.txt b/docs/specs/vhost-user.txt
> index 0062baa..c2d2e2a 100644
> --- a/docs/specs/vhost-user.txt
> +++ b/docs/specs/vhost-user.txt
> @@ -120,6 +120,7 @@ There are several messages that the master sends with file descriptors passed
>  in the ancillary data:
>  
>   * VHOST_SET_MEM_TABLE
> + * VHOST_SET_LOG_BASE (if VHOST_USER_PROTOCOL_F_LOG_SHMFD)
>   * VHOST_SET_LOG_FD
>   * VHOST_SET_VRING_KICK
>   * VHOST_SET_VRING_CALL
> @@ -135,6 +136,11 @@ As older slaves don't support negotiating protocol features,
>  a feature bit was dedicated for this purpose:
>  #define VHOST_USER_F_PROTOCOL_FEATURES 30
>  
> +Protocol features
> +-----------------
> +
> +#define VHOST_USER_PROTOCOL_F_LOG_SHMFD 0
> +
>  Message types
>  -------------
>  
> @@ -301,3 +307,37 @@ Message types
>        Bits (0-7) of the payload contain the vring index. Bit 8 is the
>        invalid FD flag. This flag is set when there is no file descriptor
>        in the ancillary data.
> +
> +Migration
> +---------
> +
> +During live migration, the master may need to track the modifications
> +the slave makes to the memory mapped regions. The client should mark
> +the dirty pages in a log. Once it complies to this logging, it may
> +declare VHOST_F_LOG_ALL has a vhost feature.
> +
> +All the modifications to memory pointed by vring "descriptor" should
> +be marked. Modifications to "used" vring should be marked if
> +VHOST_VRING_F_LOG is part of ring's features.

It's device's features I think.

> +
> +Dirty pages are of size:
> +#define VHOST_LOG_PAGE 0x1000
> +
> +The log memory fd is provided in the ancillary data of
> +VHOST_USER_SET_LOG_BASE message when the slave has
> +VHOST_USER_PROTOCOL_F_LOG_SHMFD protocol feature.
> +
> +The size of the log may be computed by using all the known guest
> +addresses. The log covers from address 0 to the maximum of guest
> +regions. In pseudo-code, to mark page at "addr" as dirty:
> +
> +page = addr / VHOST_LOG_PAGE
> +log[page / 8] |= 1 << page % 8

Pls note it must be done atomically.


> +
> +VHOST_USER_SET_LOG_FD is an optional message with an eventfd in
> +ancillary data, it may be used to inform the master that the log has
> +been modified.
> +
> +Once the source has finished migration, VHOST_USER_RESET_OWNER message
> +will be sent by the source. No further update must be done before the
> +destination takes over with new regions & rings.
> -- 
> 2.4.3