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From:   "Michael Kelley (LINUX)" <mikelley@microsoft.com>
To:     Yanming Liu <yanminglr@gmail.com>
CC:     Wei Liu <wei.liu@kernel.org>,
        Andrea Parri <parri.andrea@gmail.com>,
        "linux-hyperv@vger.kernel.org" <linux-hyperv@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "dri-devel@lists.freedesktop.org" <dri-devel@lists.freedesktop.org>,
        "linux-fbdev@vger.kernel.org" <linux-fbdev@vger.kernel.org>,
        KY Srinivasan <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        Dexuan Cui <decui@microsoft.com>,
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        "airlied@linux.ie" <airlied@linux.ie>,
        "daniel@ffwll.ch" <daniel@ffwll.ch>,
        "lkmlabelt@gmail.com" <lkmlabelt@gmail.com>
Subject: RE: [PATCH v2] hv: account for packet descriptor in maximum packet
 size
Thread-Topic: [PATCH v2] hv: account for packet descriptor in maximum packet
 size
Thread-Index: AQHYBbs9Z9MCDPVXTEy3Gdtosh0+p6xi6g+AgAe+98CAAC2igIAMpPbQ
Date:   Thu, 27 Jan 2022 21:22:34 +0000
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From: Yanming Liu <yanminglr@gmail.com> Sent: Wednesday, January 19, 2022 1=
2:14 PM
>=20
> On Thu, Jan 20, 2022 at 2:12 AM Michael Kelley (LINUX)
> <mikelley@microsoft.com> wrote:
> >
> > From: Wei Liu <wei.liu@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.
> >
>=20
> 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@liuw=
e-devbox-debian-v2/T/
>=20
> 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.
>=20

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