[PATCH v2] hv: account for packet descriptor in maximum packet size

[Michael Kelley (LINUX)]

From: Yanming Liu <yanminglr at gmail.com> Sent: Wednesday, January 19, 2022 12:14 PM
> 
> On Thu, Jan 20, 2022 at 2:12 AM Michael Kelley (LINUX)
> <mikelley at microsoft.com> wrote:
> >
> > From: Wei Liu <wei.liu at kernel.org> Sent: Friday, January 14, 2022 11:13 AM
> > >
> > > On Mon, Jan 10, 2022 at 01:44:19AM +0100, Andrea Parri wrote:
> > > > (Extending Cc: list,)
> > > >
> > > > On Sun, Jan 09, 2022 at 05:55:16PM +0800, Yanming Liu wrote:
> >
> > The VSS driver in hv_snapshot.c allocates a receive buffer of 8 Kbytes
> > and sets max_pkt_size to 8 Kbytes.  But the received messages are
> > all fixed size and small.  I don't know why the driver uses an 8 Kbyte
> > receive buffer instead of 4 Kbytes, but the current settings are
> > more than sufficient.
> >
> 
> Well, I'm not sure, on August 2021 there was a patch changing
> max_pkt_size to 8 KiB for VSS driver:
> https://lore.kernel.org/linux-hyperv/20210825190217.qh2c6yq5qr3ntum5@liuwe-devbox-debian-v2/T/
> 
> The patch mentioned a 6304 bytes VSS message. Which is part of the
> reason I tried to address the more "general" problem of potentially
> mismatching buffer size.
> 

This is certainly interesting.   The Linux driver is not processing
all those bytes, so I'm not sure what Hyper-V is passing to the
guest.  I'll check with the Hyper-V team to be sure.

Michael