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

[Haiyang Zhang]

> -----Original Message-----
> From: Lorenzo Pieralisi <lorenzo.pieralisi@xxxxxxx>
> Sent: Tuesday, August 13, 2019 10:26 AM
> To: Haiyang Zhang <haiyangz@xxxxxxxxxxxxx>
> Cc: sashal@xxxxxxxxxx; bhelgaas@xxxxxxxxxx; linux-
> hyperv@xxxxxxxxxxxxxxx; linux-pci@xxxxxxxxxxxxxxx; KY Srinivasan
> <kys@xxxxxxxxxxxxx>; Stephen Hemminger <sthemmin@xxxxxxxxxxxxx>;
> olaf@xxxxxxxxx; vkuznets <vkuznets@xxxxxxxxxx>; linux-
> kernel@xxxxxxxxxxxxxxx
> 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@xxxxxxx>
> > > Sent: Tuesday, August 13, 2019 6:14 AM
> > > To: Haiyang Zhang <haiyangz@xxxxxxxxxxxxx>
> > > Cc: sashal@xxxxxxxxxx; bhelgaas@xxxxxxxxxx; linux-
> > > hyperv@xxxxxxxxxxxxxxx; linux-pci@xxxxxxxxxxxxxxx; KY Srinivasan
> > > <kys@xxxxxxxxxxxxx>; Stephen Hemminger
> <sthemmin@xxxxxxxxxxxxx>;
> > > olaf@xxxxxxxxx; vkuznets <vkuznets@xxxxxxxxxx>; linux-
> > > kernel@xxxxxxxxxxxxxxx
> > > 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.

Sure, I agree with you, and will break the patch into two, and resubmit.

Thanks,
- Haiyang