On Mon, Jul 11, 2022 at 1:49 PM Stephen Boyd sboyd@kernel.org wrote:

Quoting Daniel Latypov (2022-07-11 09:27:13)

...

CONFIG_UML_PCI_OVER_VIRTIO=y is needed to enable CONFIG_PCI=y on UML. However, this causes test failures when running the clk tests, i.e. $ ./tools/testing/kunit/kunit.py run --kunitconfig=drivers/clk

A snippet of the particular error is:

...

ok 1 - clk_gate_test_parent_rate

------------[ cut here ]------------ WARNING: CPU: 0 PID: 45 at lib/logic_iomem.c:141 __raw_readl+0x9f/0xd0

This is triggered by this cast in the test: 143 ctx->fake_mem = (void __force __iomem *)&ctx->fake_reg; this seems to work except when logic iomem is enabled, i.e. CONFIG_INDIRECT_IOMEM=y.

As a short-term fix, explicitly disable CONFIG_UML_PCI_OVER_VIRTIO in drivers/clk/.kunitconfig so we can enable it for everyone else by default in kunit.py.

The long-term fix probably requires something more complicated, like #ifdef CONFIG_INDIRECT_IOMEM logic_iomem_add_region(...); #endif

If this is how platform IO is mocked then yes we'll have to implement that.

It's not clear that it will be, but it's likely. Until it's more clear, I figured we'd go with this simple bandaid fix for now.

There was initially some talk of using function-level "stubbing"/"mocking" [1] to do the same, but Brendan started playing around with logic iomem more. He and David can comment on it more cogently than I can.

[1] https://lore.kernel.org/linux-kselftest/20220318021314.3225240-3-davidgow@go...

Daniel

Quoting David Gow (2022-07-11 23:44:08)

So, as I understand it, casting a regular pointer to an __iomem pointer (as the clk test does) isn't technically correct, though it does work on almost every architecture out there. If we want some way of intercepting I/O access, then then that'll need to be handled by the various read()/write() functions.

Yep. It's test code though so it seemed ok at the time.

UML doesn't support iomem at all out of the box, and logic_iomem is a way of implementing it which allow us to attach handler functions to blocks of memory, albeit with more constraints about what addresses get used. Brendan started implementing a "fake hardware" interface on top of this here, though it's still in-progress: https://kunit-review.googlesource.com/c/linux/+/5272/4

Cool.

Ultimately, I think the 'correct' solution here will be logic_iomem-based, but doing that nicely will probably require one of two things:

• logic_iomem to support non-UML architectures as well (becoming a

generic "insert a 'fake' device here" system)

• logic_iomem to have some way of "passing through" access to an io

memory region through to the normal underlying memory.

Ideally, we'll have both, and maybe even defaults which will allow hacks like this to continue working (perhaps with a warning?). That'll require some significant (and probably not uncontroversial) work on how iomem accesses work in general, though, possibly with performance impact.

Does it matter to pass through to real iomem? I'd think we wouldn't want to actually affect real hardware in test code. Instead we'd like to fake it and then look at the result, like how the clk test works.

The other option of using function redirection on the io read() write() functions exists, and would be a bit simpler in the short-term, but would probably result in a lot of tests reimplementing this, and also would have some performance impacts, as the I/O accesses wouldn't be able to be inlined if KUNIT is enabled.

That sounds OK to me because nobody is enabling KUNIT in production, right?

On Wed, Jul 27, 2022 at 4:07 AM Stephen Boyd sboyd@kernel.org wrote:

Quoting David Gow (2022-07-11 23:44:08)

...

So, as I understand it, casting a regular pointer to an __iomem pointer (as the clk test does) isn't technically correct, though it does work on almost every architecture out there. If we want some way of intercepting I/O access, then then that'll need to be handled by the various read()/write() functions.

Yep. It's test code though so it seemed ok at the time.

...

UML doesn't support iomem at all out of the box, and logic_iomem is a way of implementing it which allow us to attach handler functions to blocks of memory, albeit with more constraints about what addresses get used. Brendan started implementing a "fake hardware" interface on top of this here, though it's still in-progress: https://kunit-review.googlesource.com/c/linux/+/5272/4

Cool.

...

Ultimately, I think the 'correct' solution here will be logic_iomem-based, but doing that nicely will probably require one of two things:

• logic_iomem to support non-UML architectures as well (becoming a

generic "insert a 'fake' device here" system)

• logic_iomem to have some way of "passing through" access to an io

memory region through to the normal underlying memory.

Ideally, we'll have both, and maybe even defaults which will allow hacks like this to continue working (perhaps with a warning?). That'll require some significant (and probably not uncontroversial) work on how iomem accesses work in general, though, possibly with performance impact.

Does it matter to pass through to real iomem? I'd think we wouldn't want to actually affect real hardware in test code. Instead we'd like to fake it and then look at the result, like how the clk test works.

The only case I'm worried about is where real iomem is necessary to boot to the point where tests run. It won't affect UML, but if you were (e.g.) running tests as modules on real hardware (or under qemu), and iomem was required to access the disk containing these modules, that would be a case for having both real and fake iomem supported on the same system.

Given the sheer number of things which depend on iomem in some regard, I suspect this'd affect pretty much every architecture except UML.

...

The other option of using function redirection on the io read() write() functions exists, and would be a bit simpler in the short-term, but would probably result in a lot of tests reimplementing this, and also would have some performance impacts, as the I/O accesses wouldn't be able to be inlined if KUNIT is enabled.

That sounds OK to me because nobody is enabling KUNIT in production, right?

As Daniel noted, Android is looking at enabling KUNIT in production (but tainting the system if any actual tests run). That's a pretty unusual use-case, though, so it definitely doesn't preclude people from redirecting functions, particularly if ftrace-based stubbing is used to avoid the overhead when disabled (though I can't recall if that works on ARM, so Android might be out of luck for those tests anyway...)

But yeah, I'm definitely not considering the potential performance impact a blocker here, just something to be aware of.

Either way, neither the stubbing support or the logic_iomem stuff is quite ready yet, so disabling CONFIG_UML_PCI_OVER_VIRTIO is the right solution in the meantime.

Cheers, -- David