> -----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
