Robin Murphy
2022-Sep-07 19:41 UTC
[PATCH v6 1/5] iommu: Return -EMEDIUMTYPE for incompatible domain and device/group
On 2022-09-07 18:00, Jason Gunthorpe wrote:> On Wed, Sep 07, 2022 at 03:23:09PM +0100, Robin Murphy wrote: >> On 2022-09-07 14:47, Jason Gunthorpe wrote: >>> On Wed, Sep 07, 2022 at 02:41:54PM +0200, Joerg Roedel wrote: >>>> On Mon, Aug 15, 2022 at 11:14:33AM -0700, Nicolin Chen wrote: >>>>> Provide a dedicated errno from the IOMMU driver during attach that the >>>>> reason attached failed is because of domain incompatability. EMEDIUMTYPE >>>>> is chosen because it is never used within the iommu subsystem today and >>>>> evokes a sense that the 'medium' aka the domain is incompatible. >>>> >>>> I am not a fan of re-using EMEDIUMTYPE or any other special value. What >>>> is needed here in EINVAL, but with a way to tell the caller which of the >>>> function parameters is actually invalid. >>> >>> Using errnos to indicate the nature of failure is a well established >>> unix practice, it is why we have hundreds of error codes and don't >>> just return -EINVAL for everything. >>> >>> What don't you like about it? >>> >>> Would you be happier if we wrote it like >>> >>> #define IOMMU_EINCOMPATIBLE_DEVICE xx >>> >>> Which tells "which of the function parameters is actually invalid" ? >> >> FWIW, we're now very close to being able to validate dev->iommu against >> where the domain came from in core code, and so short-circuit ->attach_dev >> entirely if they don't match. > > I don't think this is a long term direction. We have systems now with > a number of SMMU blocks and we really are going to see a need that > they share the iommu_domains so we don't have unncessary overheads > from duplicated io page table memory. > > So ultimately I'd expect to pass the iommu_domain to the driver and > the driver will decide if the page table memory it represents is > compatible or not. Restricting to only the same iommu instance isn't > good..Who said IOMMU instance? As a reminder, the patch I currently have[1] is matching the driver (via the device ops), which happens to be entirely compatible with drivers supporting cross-instance domains. Mostly because we already have drivers that support cross-instance domains and callers that use them.>> At that point -EINVAL at the driver callback level could be assumed >> to refer to the domain argument, while anything else could be taken >> as something going unexpectedly wrong when the attach may otherwise >> have worked. I've forgotten if we actually had a valid case anywhere >> for "this is my device but even if you retry with a different domain >> it's still never going to work", but I think we wouldn't actually >> need that anyway - it should be clear enough to a caller that if >> attaching to an existing domain fails, then allocating a fresh >> domain and attaching also fails, that's the point to give up. > > The point was to have clear error handling, we either have permenent > errors or 'this domain will never work with this device error'. > > If we treat all error as temporary and just retry randomly it can > create a mess. For instance we might fail to attach to a perfectly > compatible domain due to ENOMEM or something and then go on to > successfully a create a new 2nd domain, just due to races. > > We can certainly code the try everything then allocate scheme, it is > just much more fragile than having definitive error codes.Again, not what I was suggesting. In fact the nature of iommu_attach_group() already rules out bogus devices getting this far, so all a driver currently has to worry about is compatibility of a device that it definitely probed with a domain that it definitely allocated. Therefore, from a caller's point of view, if attaching to an existing domain returns -EINVAL, try another domain; multiple different existing domains can be tried, and may also return -EINVAL for the same or different reasons; the final attempt is to allocate a fresh domain and attach to that, which should always be nominally valid and *never* return -EINVAL. If any attempt returns any other error, bail out down the usual "this should have worked but something went wrong" path. Even if any driver did have a nonsensical "nothing went wrong, I just can't attach my device to any of my domains" case, I don't think it would really need distinguishing from any other general error anyway. Once multiple drivers are in play, the only addition is that the "gatekeeper" check inside iommu_attach_group() may also return -EINVAL if the device is managed by a different driver, since that still fits the same "try again with a different domain" message to the caller. It's actually quite neat - basically the exact same thing we've tried to do with -EMEDIUMTYPE here, but more self-explanatory, since the fact is that a domain itself should never be invalid for attaching to via its own ops, and a group should never be inherently invalid for attaching to a suitable domain, it is only ever a particular combination of group (or device at the internal level) and domain that may not be valid together. Thus as long as we can maintain that basic guarantee that attaching a group to a newly allocated domain can only ever fail for resource allocation reasons and not some spurious "incompatibility", then we don't need any obscure trickery, and a single, clear, error code is in fact enough to say all that needs to be said. Whether iommu_attach_device() should also join the party and start rejecting non-singleton-group devices with a different error, or maintain its current behaviour since its legacy users already have their expectations set, is another matter in its own right. Cheers, Robin. [1] https://gitlab.arm.com/linux-arm/linux-rm/-/commit/683cdff1b2d4ae11f56e38d93b37e66e8c939fc9