linux-kernel - RE: [PATCH v2] hv: account for packet descriptor in maximum packet size

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Message-ID: <MWHPR21MB1593F5A0494D7726E577E03DD7219@MWHPR21MB1593.namprd21.prod.outlook.com>
Date:   Thu, 27 Jan 2022 21:22:34 +0000
From:   "Michael Kelley (LINUX)" <mikelley@...rosoft.com>
To:     Yanming Liu <yanminglr@...il.com>
CC:     Wei Liu <wei.liu@...nel.org>,
        Andrea Parri <parri.andrea@...il.com>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "linux-fbdev@...r.kernel.org" <linux-fbdev@...r.kernel.org>,
        KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Dexuan Cui <decui@...rosoft.com>,
        "drawat.floss@...il.com" <drawat.floss@...il.com>,
        "airlied@...ux.ie" <airlied@...ux.ie>,
        "daniel@...ll.ch" <daniel@...ll.ch>,
        "lkmlabelt@...il.com" <lkmlabelt@...il.com>
Subject: RE: [PATCH v2] hv: account for packet descriptor in maximum packet
 size

From: Yanming Liu <yanminglr@...il.com> Sent: Wednesday, January 19, 2022 12:14 PM
> 
> On Thu, Jan 20, 2022 at 2:12 AM Michael Kelley (LINUX)
> <mikelley@...rosoft.com> wrote:
> >
> > From: Wei Liu <wei.liu@...nel.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