-----Original Message-----
From: Lorenzo Pieralisi <redacted>
Sent: Monday, August 12, 2019 11:39 AM
To: Haiyang Zhang <haiyangz@microsoft.com>
Cc: sashal@kernel.org; bhelgaas@google.com; linux-
hyperv@vger.kernel.org; linux-pci@vger.kernel.org; KY Srinivasan
[off-list ref]; Stephen Hemminger [off-list ref];
olaf@aepfle.de; vkuznets [off-list ref]; linux-
kernel@vger.kernel.org
Subject: Re: [PATCH v2] PCI: hv: Detect and fix Hyper-V PCI domain number
collision

On Tue, Aug 06, 2019 at 11:52:11PM +0000, Haiyang Zhang wrote:

quoted

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 is used as PCI domain number. So, as recommended by Azure host team,
we now use the bytes 4 and 5 which usually contain unique numbers as PCI
domain. 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.

This is not clear at all. Why "finding another number" is fine with
this patch while it is not with current kernel code ? Also does this
have backward compatibility issues ?

The bytes 4, 5 have more uniqueness (info entropy) than bytes 8, 9, so we use
bytes 4, 5. On older hosts, bytes 4, 5 can also be used -- so it has no backward
compatibility issues.
 

I do not understand if a collision is a problem or not from the
log above.

Collision will cause the second device with the same domain number fails to load.
I will include these info into the patch description.

quoted

Thanks to Michael Kelley [off-list ref] for proposing this

idea.

Add it as Suggested-by: tag.

I will add this line.

Thanks,
- Haiyang