linux-kernel - Re: [PATCH v3] PCI: hv: Detect and fix Hyper-V PCI domain number collision

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Message-ID: <20190813142540.GA5070@e121166-lin.cambridge.arm.com>
Date:   Tue, 13 Aug 2019 15:25:40 +0100
From:   Lorenzo Pieralisi <lorenzo.pieralisi@....com>
To:     Haiyang Zhang <haiyangz@...rosoft.com>
Cc:     "sashal@...nel.org" <sashal@...nel.org>,
        "bhelgaas@...gle.com" <bhelgaas@...gle.com>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        KY Srinivasan <kys@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "olaf@...fle.de" <olaf@...fle.de>, vkuznets <vkuznets@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] PCI: hv: Detect and fix Hyper-V PCI domain number
 collision

On Tue, Aug 13, 2019 at 12:55:59PM +0000, Haiyang Zhang wrote:
> 
> 
> > -----Original Message-----
> > From: Lorenzo Pieralisi <lorenzo.pieralisi@....com>
> > Sent: Tuesday, August 13, 2019 6:14 AM
> > To: Haiyang Zhang <haiyangz@...rosoft.com>
> > Cc: sashal@...nel.org; bhelgaas@...gle.com; linux-
> > hyperv@...r.kernel.org; linux-pci@...r.kernel.org; KY Srinivasan
> > <kys@...rosoft.com>; Stephen Hemminger <sthemmin@...rosoft.com>;
> > olaf@...fle.de; vkuznets <vkuznets@...hat.com>; linux-
> > kernel@...r.kernel.org
> > Subject: Re: [PATCH v3] PCI: hv: Detect and fix Hyper-V PCI domain number
> > collision
> > 
> > On Mon, Aug 12, 2019 at 06:20:53PM +0000, Haiyang Zhang wrote:
> > > Currently in Azure cloud, for passthrough devices including GPU, the host
> > > sets the device instance ID's bytes 8 - 15 to a value derived from the host
> > > HWID, which is the same on all devices in a VM. So, the device instance
> > > ID's bytes 8 and 9 provided by the host are no longer unique. This can
> > > cause device passthrough to VMs to fail because the bytes 8 and 9 are used
> > > as PCI domain number. Collision of domain numbers will cause the second
> > > device with the same domain number fail to load.
> > >
> > > As recommended by Azure host team, the bytes 4, 5 have more uniqueness
> > > (info entropy) than bytes 8, 9. So now we use bytes 4, 5 as the PCI domain
> > > numbers. On older hosts, bytes 4, 5 can also be used -- no backward
> > > compatibility issues here. The chance of collision is greatly reduced. In
> > > the rare cases of collision, we will detect and find another number that is
> > > not in use.
> > 
> > I have not explained what I meant correctly. This patch fixes an
> > issue and the "find another number" fallback can be also applied
> > to the current kernel without changing the bytes you use for
> > domain numbers.
> > 
> > This patch would leave old kernels susceptible to breakage.
> > 
> > Again, I have no Azure knowledge but it seems better to me to
> > add a fallback "find another number" allocation on top of mainline
> > and send it to stable kernels. Then we can add another patch to
> > change the bytes you use to reduce the number of collision.
> > 
> > Please let me know what you think, thanks.
> 
> Thanks for your clarification.
> Actually, I hope the stable kernel will be patched to use bytes 4,5 too,
> because host provided numbers are persistent across reboots, we like
> to use them if possible.
> 
> I think we can either --
> 1) Apply this patch for mainline and stable kernels as well.
> 2) Or, break this patch into two patches, and apply both of them for 
> Mainline and stable kernels.

(2) since one patch is a fix and the other one an (optional - however
important it is) change.

This way if the optional change needs reverting we still have a working
kernel.

In the end it is up to you - I am just expressing what I think is the
most sensible way forward.

Lorenzo