Re: [libvirt] [Qemu-devel] [PATCH 1/2] vfio/mdev: add version field as mandatory attribute for mdev device

On Tue, Apr 23, 2019 at 09:21:00AM +0800, Alex Williamson wrote: > On Mon, 22 Apr 2019 21:01:52 -0400 > Yan Zhao <yan.y.zhao(a)intel.com&gt; wrote: > > > On Mon, Apr 22, 2019 at 10:39:50PM +0800, Alex Williamson wrote: > > > On Fri, 19 Apr 2019 04:35:04 -0400 > > > Yan Zhao <yan.y.zhao(a)intel.com&gt; wrote: > > > > > > > device version attribute in mdev sysfs is used by user space software > > > > (e.g. libvirt) to query device compatibility for live migration of VFIO > > > > mdev devices. This attribute is mandatory if a mdev device supports live > > > > migration. > > > > > > The Subject: doesn't quite match what's being proposed here. > > > > > > > It consists of two parts: common part and vendor proprietary part. > > > > common part: 32 bit. lower 16 bits is vendor id and higher 16 bits > > > > identifies device type. e.g., for pci device, it is > > > > "pci vendor id" | (VFIO_DEVICE_FLAGS_PCI << 16). > > > > > > What purpose does this serve? If it's intended as some sort of > > > namespace feature, shouldn't we first assume that we can only support > > > migration to devices of the same type? Therefore each type would > > > already have its own namespace. Also that would make the trailing bit > > > of the version string listed below in the example redundant. A vendor > > > is still welcome to include this in their version string if they wish, > > > but I think the string should be entirely vendor defined. > > > > > hi Alex, > > This common part is a kind of namespace. > > Because if version string is entirely defined by vendors, I'm worried about > > if there is a case that one vendor's version string happens to deceive and > > interfere with another vendor's version checking? > > e.g. > > vendor A has a version string like: vendor id + device id + mdev type > > vendor B has a version string like: device id + vendor id + mdev type > > but vendor A's vendor id is 0x8086, device id is 0x1217 > > vendor B's vendor id is 0x1217, device id is 0x8086. > > > > In this corner case, the two vendors may regard the two device is > > migratable but actually they are not. > > > > That's the reason for this common part that serve as a kind of namespace > > that all vendors will comply with to avoid overlap. > > If we assume that migration can only occur between matching mdev types, > this is redundant, each type already has their own namespace. > hi Alex, do you mean user space software like libvirt needs to first check whether mdev type is matching and then check whether version is matching?