Re: [Qemu-devel] [PATCH] vhost-user: update spec description

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

On Mon, Nov 16, 2015 at 11:36:47AM +0200, Victor Kaplansky wrote:
> On Sun, Nov 15, 2015 at 09:29:59PM +0200, Michael S. Tsirkin wrote:
> > Clarify logging setup to make sure all clients comply in a way that is
> > future-proof.  Document how rings are started/stopped.
> > 
> > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > ---
> >  docs/specs/vhost-user.txt | 62 ++++++++++++++++++++++++++++++++++++++++-------
> >  1 file changed, 53 insertions(+), 9 deletions(-)
> > 
> > diff --git a/docs/specs/vhost-user.txt b/docs/specs/vhost-user.txt
> > index 26dde2e..6a0ed8f 100644
> > --- a/docs/specs/vhost-user.txt
> > +++ b/docs/specs/vhost-user.txt
> > @@ -87,6 +87,14 @@ Depending on the request type, payload can be:
> >     User address: a 64-bit user address
> >     mmap offset: 64-bit offset where region starts in the mapped memory
> >  
> > +* Log description
> > +   ---------------------------
> > +   | log size | log offset |
> > +   ---------------------------
> > +   log size: size of area used for logging
> > +   log offset: offset from start of supplied file descriptor
> > +       where logging starts (i.e. where guest address 0 would be logged)
> > +
> >  In QEMU the vhost-user message is implemented with the following struct:
> >  
> >  typedef struct VhostUserMsg {
> > @@ -138,6 +146,23 @@ As older slaves don't support negotiating protocol features,
> >  a feature bit was dedicated for this purpose:
> >  #define VHOST_USER_F_PROTOCOL_FEATURES 30
> >  
> > +Starting and stopping rings
> > +----------------------
> > +Each ring is initialized in a stopped state, client must not process it until
> > +ring is enabled.
> > +
> > +If VHOST_USER_F_PROTOCOL_FEATURES has been negotiated, client must start and
> > +stop ring processing upon receiving VHOST_USER_SET_VRING_ENABLE with parameters
> > +1 and 0 respoectively.
> > +
> > +If VHOST_USER_F_PROTOCOL_FEATURES has not been negotiated, client must start
> > +ring processing upon receiving a kick (that is, detecting that file descriptor
> > +is readable) on the descriptor specified by VHOST_USER_SET_VRING_KICK, and stop
> > +ring processing upon receiving VHOST_USER_GET_VRING_BASE.
> > +
> > +While rings are running, client must support changing some configuration
> > +aspects on the fly.
> > +
> >  Multiple queue support
> >  ----------------------
> >  
> > @@ -162,9 +187,13 @@ 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 the VHOST_F_LOG_ALL vhost feature.
> >  
> > +To start/stop logging of data/used ring writes, server may send messages
> > +VHOST_USER_SET_FEATURES with VHOST_F_LOG_ALL and VHOST_USER_SET_VRING_ADDR with
> > +VHOST_VRING_F_LOG in ring's flags set to 1/0, respectively.
> > +
> >  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.
> > +VHOST_VRING_F_LOG is part of ring's flags.
> >  
> >  Dirty pages are of size:
> >  #define VHOST_LOG_PAGE 0x1000
> > @@ -173,22 +202,35 @@ 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
> > +The size of the log is supplied as part of VhostUserMsg
> > +which should be large enough to cover all known guest
> > +addresses. Log starts at the supplied offset in the
> > +supplied file descriptor.
> > +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
> >  
> > +Where addr is the guest physical address.
> > +
> >  Use atomic operations, as the log may be concurrently manipulated.
> >  
> > +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
> > +log start. Also note that this value might be outside the legal guest physical
> > +address range (i.e. does not have to be covered by the VhostUserMemory table),
> > +but the bit offset of the last byte of the ring must fall within
> > +the size supplied by VhostUserLog.
> > +
> >  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.
> > +Once the source has finished migration, rings will be stopped by
> > +the source. No further update must be done before rings are
> > +restarted.
> >  
> >  Protocol features
> >  -----------------
> > @@ -259,11 +301,11 @@ Message types
> >   * VHOST_USER_RESET_OWNER
> >  
> >        Id: 4
> > -      Equivalent ioctl: VHOST_RESET_OWNER
> >        Master payload: N/A
> >  
> > -      Issued when a new connection is about to be closed. The Master will no
> > -      longer own this connection (and will usually close it).
> > +      This is no longer used. Used to be sent to request stopping
> > +      all rings, but some clients interpreted it to also discard
> > +      connection state (this interpretation would lead to bugs).
> 
> The new code uses VHOST_USER_RESET_DEVICE as the name for request
> Id 4.

This was reverted before rc0.

>  Also it is not clear whether it is recommended just to
> ignore the reset device request now?

Since it's never sent, does it matter?

> >  
> >   * VHOST_USER_SET_MEM_TABLE
> >  
> > @@ -388,6 +430,8 @@ Message types
> >        Master payload: vring state description
> >  
> >        Signal slave to enable or disable corresponding vring.
> > +      This request should be sent only when VHOST_USER_F_PROTOCOL_FEATURES
> > +      has been negotiated.
> >  
> >   * VHOST_USER_SEND_RARP
> >  
> > -- 
> > MST
> --Victor