From: Robin Murphy <robin.murphy@arm.com>
To: Jason Gunthorpe <jgg@nvidia.com>, Joerg Roedel <joro@8bytes.org>
Cc: Nicolin Chen <nicolinc@nvidia.com>,
will@kernel.org, alex.williamson@redhat.com,
suravee.suthikulpanit@amd.com, marcan@marcan.st,
sven@svenpeter.dev, alyssa@rosenzweig.io, robdclark@gmail.com,
dwmw2@infradead.org, baolu.lu@linux.intel.com,
mjrosato@linux.ibm.com, gerald.schaefer@linux.ibm.com,
orsonzhai@gmail.com, baolin.wang@linux.alibaba.com,
zhang.lyra@gmail.com, thierry.reding@gmail.com,
vdumpa@nvidia.com, jonathanh@nvidia.com,
jean-philippe@linaro.org, cohuck@redhat.com, tglx@linutronix.de,
shameerali.kolothum.thodi@huawei.com, thunder.leizhen@huawei.com,
christophe.jaillet@wanadoo.fr, yangyingliang@huawei.com,
jon@solid-run.com, iommu@lists.linux.dev,
linux-kernel@vger.kernel.org, asahi@lists.linux.dev,
linux-arm-kernel@lists.infradead.org,
linux-arm-msm@vger.kernel.org, linux-s390@vger.kernel.org,
linux-tegra@vger.kernel.org,
virtualization@lists.linux-foundation.org, kvm@vger.kernel.org,
kevin.tian@intel.com
Subject: Re: [PATCH v6 1/5] iommu: Return -EMEDIUMTYPE for incompatible domain and device/group
Date: Thu, 8 Sep 2022 11:25:46 +0100 [thread overview]
Message-ID: <7ef259b2-121e-643e-49c2-0b65923d392d@arm.com> (raw)
In-Reply-To: <Yxk6sR4JiAAn3Jf5@nvidia.com>
On 2022-09-08 01:43, Jason Gunthorpe wrote:
> On Wed, Sep 07, 2022 at 08:41:13PM +0100, Robin Murphy wrote:
>
>>>> 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?
>
> Ah, I completely misunderstood what 'dev->iommu' was referring too, OK
> I see.
>
>> 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.
>
> The algorithm you described is exactly what this series does, it just
> used EMEDIUMTYPE instead of EINVAL. Changing it to EINVAL is not a
> fundamental problem, just a bit more work.
>
> Looking at Nicolin's series there is a bunch of existing errnos that
> would still need converting, ie EXDEV, EBUSY, EOPNOTSUPP, EFAULT, and
> ENXIO are all returned as codes for 'domain incompatible with device'
> in various drivers. So the patch would still look much the same, just
> changing them to EINVAL instead of EMEDIUMTYPE.
>
> That leaves the question of the remaining EINVAL's that Nicolin did
> not convert to EMEDIUMTYPE.
>
> eg in the AMD driver:
>
> if (!check_device(dev))
> return -EINVAL;
>
> iommu = rlookup_amd_iommu(dev);
> if (!iommu)
> return -EINVAL;
>
> These are all cases of 'something is really wrong with the device or
> iommu, everything will fail'. Other drivers are using ENODEV for this
> already, so we'd probably have an additional patch changing various
> places like that to ENODEV.
>
> This mixture of error codes is the basic reason why a new code was
> used, because none of the existing codes are used with any
> consistency.
>
> But OK, I'm on board, lets use more common errnos with specific
> meaning, that can be documented in a comment someplace:
> ENOMEM - out of memory
> ENODEV - no domain can attach, device or iommu is messed up
> EINVAL - the domain is incompatible with the device
> <others> - Same behavior as ENODEV, use is discouraged.
>
> I think achieving consistency of error codes is a generally desirable
> goal, it makes the error code actually useful.
>
> Joerg this is a good bit of work, will you be OK with it?
>
>> 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.
>
> As above, this is not the case, drivers do seem to have error paths
> that are unconditional on the domain. Perhaps they are just protective
> assertions and never happen.
Right, that's the gist of what I was getting at - I think it's worth
putting in the effort to audit and fix the drivers so that that *can* be
the case, then we can have a meaningful error API with standard codes
effectively for free, rather than just sighing at the existing mess and
building a slightly esoteric special case on top.
Case in point, the AMD checks quoted above are pointless, since it
checks the same things in ->probe_device, and if that fails then the
device won't get a group so there's no way for it to even reach
->attach_dev any more. I'm sure there's a *lot* of cruft that can be
cleared out now that per-device and per-domain ops give us this kind of
inherent robustness.
Cheers,
Robin.
> Regardless, it doesn't matter. If they return ENODEV or EINVAL the
> VFIO side algorithm will continue to work fine, it just does alot more
> work if EINVAL is permanently returned.
>
> Thanks,
> Jason
next prev parent reply other threads:[~2022-09-08 10:26 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-08-15 18:14 [PATCH v6 0/5] Simplify vfio_iommu_type1 attach/detach routine Nicolin Chen
2022-08-15 18:14 ` [PATCH v6 1/5] iommu: Return -EMEDIUMTYPE for incompatible domain and device/group Nicolin Chen
2022-09-07 12:41 ` Joerg Roedel
2022-09-07 13:47 ` Jason Gunthorpe
2022-09-07 14:06 ` Joerg Roedel
2022-09-07 17:10 ` Jason Gunthorpe
2022-09-08 13:28 ` Joerg Roedel
2022-09-08 16:14 ` Jason Gunthorpe
2022-09-09 3:17 ` Nicolin Chen
2022-09-09 5:00 ` Tian, Kevin
2022-09-09 12:07 ` Jason Gunthorpe
2022-09-13 2:22 ` Tian, Kevin
2022-09-13 5:07 ` Nicolin Chen
2022-09-07 14:23 ` Robin Murphy
2022-09-07 17:00 ` Jason Gunthorpe
2022-09-07 19:41 ` Robin Murphy
2022-09-08 0:43 ` Jason Gunthorpe
2022-09-08 9:30 ` Tian, Kevin
2022-09-08 12:08 ` Jason Gunthorpe
2022-09-10 23:35 ` Nicolin Chen
2022-09-13 2:24 ` Tian, Kevin
2022-09-13 8:36 ` Nicolin Chen
2022-09-08 9:54 ` Tian, Kevin
2022-09-08 10:25 ` Robin Murphy [this message]
2022-08-15 18:14 ` [PATCH v6 2/5] vfio/iommu_type1: Prefer to reuse domains vs match enforced cache coherency Nicolin Chen
2022-08-15 18:14 ` [PATCH v6 3/5] vfio/iommu_type1: Remove the domain->ops comparison Nicolin Chen
2022-08-15 18:14 ` [PATCH v6 4/5] vfio/iommu_type1: Clean up update_dirty_scope in detach_group() Nicolin Chen
2022-08-15 18:14 ` [PATCH v6 5/5] vfio/iommu_type1: Simplify group attachment Nicolin Chen
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