Re: [libvirt] [PATCH v2 1/2] vfio/mdev: add version attribute for mdev device

* Cornelia Huck (cohuck(a)redhat.com) wrote: > On Thu, 9 May 2019 17:48:26 +0100 > "Dr. David Alan Gilbert" <dgilbert(a)redhat.com&gt; wrote: > > > * Cornelia Huck (cohuck(a)redhat.com) wrote: > > > On Thu, 9 May 2019 16:48:57 +0100 > > > "Dr. David Alan Gilbert" <dgilbert(a)redhat.com&gt; wrote: > > > > > > > * Cornelia Huck (cohuck(a)redhat.com) wrote: > > > > > On Tue, 7 May 2019 15:18:26 -0600 > > > > > Alex Williamson <alex.williamson(a)redhat.com&gt; wrote: > > > > > > > > > > > On Sun, 5 May 2019 21:49:04 -0400 > > > > > > Yan Zhao <yan.y.zhao(a)intel.com&gt; wrote: > > > > > > > > > > > > + Errno: > > > > > > > + If vendor driver wants to claim a mdev device incompatible to all other mdev > > > > > > > + devices, it should not register version attribute for this mdev device. But if > > > > > > > + a vendor driver has already registered version attribute and it wants to claim > > > > > > > + a mdev device incompatible to all other mdev devices, it needs to return > > > > > > > + -ENODEV on access to this mdev device's version attribute. > > > > > > > + If a mdev device is only incompatible to certain mdev devices, write of > > > > > > > + incompatible mdev devices's version strings to its version attribute should > > > > > > > + return -EINVAL; > > > > > > > > > > > > I think it's best not to define the specific errno returned for a > > > > > > specific situation, let the vendor driver decide, userspace simply > > > > > > needs to know that an errno on read indicates the device does not > > > > > > support migration version comparison and that an errno on write > > > > > > indicates the devices are incompatible or the target doesn't support > > > > > > migration versions. > > > > > > > > > > I think I have to disagree here: It's probably valuable to have an > > > > > agreed error for 'cannot migrate at all' vs 'cannot migrate between > > > > > those two particular devices'. Userspace might want to do different > > > > > things (e.g. trying with different device pairs). > > > > > > > > Trying to stuff these things down an errno seems a bad idea; we can't > > > > get much information that way. > > > > > > So, what would be a reasonable approach? Userspace should first read > > > the version attributes on both devices (to find out whether migration > > > is supported at all), and only then figure out via writing whether they > > > are compatible? > > > > > > (Or just go ahead and try, if it does not care about the reason.) > > > > Well, I'm OK with something like writing to test whether it's > > compatible, it's just we need a better way of saying 'no'. > > I'm not sure if that involves reading back from somewhere after > > the write or what. > > Hm, so I basically see two ways of doing that: > - standardize on some error codes... problem: error codes can be hard > to fit to reasons > - make the error available in some attribute that can be read > > I'm not sure how we can serialize the readback with the last write, > though (this looks inherently racy). > > How important is detailed error reporting here? I think we need something, otherwise we're just going to get vague user reports of 'but my VM doesn't migrate'; I'd like the error to be good enough to point most users to something they can understand (e.g. wrong card family/too old a driver etc).