From:   Haiyang Zhang <haiyangz@microsoft.com>
To:     Francois Romieu <romieu@fr.zoreil.com>
CC:     "linux-hyperv@vger.kernel.org" <linux-hyperv@vger.kernel.org>,
        "netdev@vger.kernel.org" <netdev@vger.kernel.org>,
        Dexuan Cui <decui@microsoft.com>,
        KY Srinivasan <kys@microsoft.com>,
        Paul Rosswurm <paulros@microsoft.com>,
        "olaf@aepfle.de" <olaf@aepfle.de>,
        "vkuznets@redhat.com" <vkuznets@redhat.com>,
        "davem@davemloft.net" <davem@davemloft.net>,
        "wei.liu@kernel.org" <wei.liu@kernel.org>,
        "edumazet@google.com" <edumazet@google.com>,
        "kuba@kernel.org" <kuba@kernel.org>,
        "pabeni@redhat.com" <pabeni@redhat.com>,
        "leon@kernel.org" <leon@kernel.org>,
        Long Li <longli@microsoft.com>,
        "ssengar@linux.microsoft.com" <ssengar@linux.microsoft.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: RE: [PATCH net-next] net: mana: Add support for jumbo frame
Thread-Topic: [PATCH net-next] net: mana: Add support for jumbo frame
Thread-Index: AQHZWqm9UOpmxqzZvUOZ7L83A2HDza8CtAUAgAAu18CAAKdXgIAAPNRg
Date:   Mon, 20 Mar 2023 15:13:45 +0000
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> -----Original Message-----
> From: Francois Romieu <romieu@fr.zoreil.com>
> Sent: Monday, March 20, 2023 7:33 AM
> To: Haiyang Zhang <haiyangz@microsoft.com>
> Cc: linux-hyperv@vger.kernel.org; netdev@vger.kernel.org; Dexuan Cui
> <decui@microsoft.com>; KY Srinivasan <kys@microsoft.com>; Paul Rosswurm
> <paulros@microsoft.com>; olaf@aepfle.de; vkuznets@redhat.com;
> davem@davemloft.net; wei.liu@kernel.org; edumazet@google.com;
> kuba@kernel.org; pabeni@redhat.com; leon@kernel.org; Long Li
> <longli@microsoft.com>; ssengar@linux.microsoft.com; linux-
> kernel@vger.kernel.org
> Subject: Re: [PATCH net-next] net: mana: Add support for jumbo frame
>=20
> [You don't often get email from romieu@fr.zoreil.com. Learn why this is
> important at https://aka.ms/LearnAboutSenderIdentification ]
>=20
> Haiyang Zhang <haiyangz@microsoft.com> :
> > > From: Francois Romieu <romieu@fr.zoreil.com>
> [...]
> > > I do not see where the driver could depend on the MTU. Even if it fai=
ls,
> > > a single call to mana_change_mtu should thus never wreck the old work=
ing
> > > state/configuration.
> > >
> > > Stated differently, the detach/attach implementation is simple but
> > > it makes the driver less reliable than it could be.
> > >
> > > No ?
> >
> > No, it doesn't make the driver less reliable. To safely remove and real=
locate
> > DMA buffers with different size, we have to stop the traffic. So,
> mana_detach()
> > is called. We also call mana_detach() in mana_close(). So the process i=
n
> > mana_change_mtu() is no more risky than ifdown/ifup of the NIC.
> >
> > In some rare cases, if the system memory is running really low, the big=
ger
> > buffer allocation may fail, so we re-try with the previous MTU. I don't=
 expect
> > it to fail again. But we still check & log the error code for completen=
ess and
> > debugging.
>=20
> In a ideal world, I would expect change_mtu() to allocate the new resourc=
es,
> bail out if some allocation fails, stop the traffic, swap the old and new
> resources, then restart the traffic and release the old resources.
> This way the device is never left in a failed state.

Thanks for the idea -- I will look into that. But the real world implementa=
tion may
be less than ideal :)

- Haiyang