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

On Tue, 14 May 2019 09:43:44 +0200 Erik Skultety <eskultet(a)redhat.com&gt; wrote: > On Tue, May 14, 2019 at 03:32:19AM -0400, Yan Zhao wrote: > > On Tue, May 14, 2019 at 03:20:40PM +0800, Erik Skultety wrote: > > > On Tue, May 14, 2019 at 02:12:35AM -0400, Yan Zhao wrote: > > > > 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? > > > > > > In a sense, yes. While it is highly desirable to have logs with descriptive > > > messages which will help in troubleshooting tremendously, I wanted to point out > > > that spending time with error logs may not be that worthwhile especially since > > > most apps (like libvirt) will solely rely on using read(3)/write(3) to sysfs. > > > That means that we're limited by the errnos available, so apart from > > > reporting the generic system message we can't any more magic in terms of the > > > error messages, so the driver needs to assure that a proper message is > > > propagated to the journal and at best libvirt can direct the user (consumer) to > > > look through the system logs for more info. I also agree with the point > > > mentioned above that defining a specific errno is IMO not the way to go, as > > > these would be just too specific for the read(3)/write(3) use case. > > > > > > That said, from libvirt POV as a consumer, I'd expect there to be truly only 2 > > > errors (I believe Alex has mentioned something similar in one of his responses > > > in one of the threads): > > > a) read error indicating that an mdev type doesn't support migration > > > - I assume if one type doesn't support migration, none of the other > > > types exposed on the parent device do, is that a fair assumption? I'd prefer not to make this assumption. Let's leave open the possibility that (for whatever reason) a vendor may choose to support migration on some types, but not others. > > > b) write error indicating that the mdev types are incompatible for > > > migration > > > > > > Regards, > > > Erik > > Thanks for this explanation. > > so, can we arrive at below agreements? > > > > 1. "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. " > > 2. vendor driver should log detailed error reasons in kernel log. > > That would be my take on this, yes, but I open to hear any other suggestions and > ideas I couldn't think of as well. Kernel logging tends to be rather ineffective, it's surprisingly difficult to get users to look in dmesg and it's not really a good choice for scraping diagnostic information either. I'd probably leave this to vendor driver's discretion at this point. Thanks, Alex

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