RE: [PATCH] hv: do not lose pending heartbeat vmbus packets

[Long Li]

> -----Original Message-----
> From: Vitaly Kuznetsov [mailto:vkuznets@xxxxxxxxxx]
> Sent: Thursday, September 29, 2016 2:22 AM
> To: KY Srinivasan <kys@xxxxxxxxxxxxx>; Long Li <longli@xxxxxxxxxxxxx>
> Cc: Haiyang Zhang <haiyangz@xxxxxxxxxxxxx>;
> devel@xxxxxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
> Subject: Re: [PATCH] hv: do not lose pending heartbeat vmbus packets
> 
> Long Li <longli@xxxxxxxxxxxxxxxxxxxxxx> writes:
> 
> > From: Long Li <longli@xxxxxxxxxxxxx>
> >
> > The host keeps sending heartbeat packets independent of guest
> responding to them. In some situations, there might be multiple heartbeat
> packets pending in the ring buffer. Don't lose them, read them all.
> >
> > Signed-off-by: Long Li <longli@xxxxxxxxxxxxx>
> 
> Long, K. Y.,
> 
> it seems this patch didn't make it to char-misc tree and it looks like an
> important fix. A couple of nitpicks below,
> 
> > ---
> >  drivers/hv/hv_util.c | 10 +++++++---
> >  1 file changed, 7 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/hv/hv_util.c b/drivers/hv/hv_util.c index
> > d5acaa2..9dc6372 100644
> > --- a/drivers/hv/hv_util.c
> > +++ b/drivers/hv/hv_util.c
> > @@ -283,10 +283,14 @@ static void heartbeat_onchannelcallback(void
> *context)
> >  	u8 *hbeat_txf_buf = util_heartbeat.recv_buffer;
> >  	struct icmsg_negotiate *negop = NULL;
> >
> > -	vmbus_recvpacket(channel, hbeat_txf_buf,
> > -			 PAGE_SIZE, &recvlen, &requestid);
> > +	while (1) {
> > +
> > +		vmbus_recvpacket(channel, hbeat_txf_buf,
> > +				 PAGE_SIZE, &recvlen, &requestid);
> 
> We should check vmbus_recvpacket() return value as well. E.g.
> hv_ringbuffer_read() may return -EAGAIN in case we didn't receive the
> whole packet (and we do this check in other drivers, see
> storvsc_on_channel_callback() for example).

I agree with you,  we should check for -EAGAIN. This should also be done in storvsc_on_channel_callback.

I think the chance of  hv_ringbuffer_read() returning -EAGAIN is almost zero. Because read_index and write_index are updated after the whole packet is written to the ring buffer, and protected by memory barriers. So getting a partial read is impossible, unless the host is doing something wrong.

Checking for recvlen is safe, because it's always set to 0 at the beginning of hv_ringbuffer_read().

Anyway, we should check for -EAGAIN for all hyperv drivers on read. I think this is a separate issue on how we deal with a buggy host. Will send another set of patches .

> 
> > +
> > +		if (!recvlen)
> 
> so this should be 'if (ret || !recvlen)'
> 
> > +			break;
> >
> > -	if (recvlen > 0) {
> >  		icmsghdrp = (struct icmsg_hdr *)&hbeat_txf_buf[
> >  				sizeof(struct vmbuspipe_hdr)];
> 
> --
>   Vitaly