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

On Mon, May 13, 2019 at 09:28:04PM +0800, Erik Skultety wrote: > On Fri, May 10, 2019 at 11:48:38AM +0200, Cornelia Huck wrote: > > On Fri, 10 May 2019 10:36:09 +0100 > > "Dr. David Alan Gilbert" <dgilbert(a)redhat.com&gt; wrote: > > > > > * 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). > > > > Ok, that sounds like a reasonable point. Not that I have a better idea > > how to achieve that, though... we could also log a more verbose error > > message to the kernel log, but that's not necessarily where a user will > > look first. > > In case of libvirt checking the compatibility, it won't matter how good the > error message in the kernel log is and regardless of how many error states you > want to handle, libvirt's only limited to errno here, since we're going to do > plain read/write, so our internal error message returned to the user is only > going to contain what the errno says - okay, of course we can (and we DO) > provide libvirt specific string, further specifying the error but like I > mentioned, depending on how many error cases we want to distinguish this may be > hard for anyone to figure out solely on the error code, as apps will most > probably not parse the > logs. > > Regards, > Erik hi Erik do you mean you are agreeing on defining common errors and only returning errno?