linux-kernel - Re: [PATCH v4,1/2] PCI: hv: Detect and fix Hyper-V PCI domain number collision

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Date:   Tue, 13 Aug 2019 23:34:28 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Haiyang Zhang <haiyangz@...rosoft.com>
Cc:     "sashal@...nel.org" <sashal@...nel.org>,
        "lorenzo.pieralisi@....com" <lorenzo.pieralisi@....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 v4,1/2] PCI: hv: Detect and fix Hyper-V PCI domain number
 collision

Thanks for splitting these; I think that makes more sense.

On Wed, Aug 14, 2019 at 12:38:54AM +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.

I think this patch is fine.  I could be misunderstanding the commit
log, but when you say "the ID bytes 8 and 9 are *no longer* unique",
that suggests that they *used* to be unique but stopped being unique
at some point, which of course raises the question of *when* they
became non-unique.

The specific information about that point would be useful to have in
the commit log, e.g., is this related to a specific version of Azure,
a configuration change, etc?

Does this problem affect GPUs more than other passthrough devices?  If
all passthrough devices are affected, why mention GPUs in particular?
I can't tell whether that information is relevant or superfluous.

Bjorn