Re: [PATCH] arch/x86: reset MXCSR to default in kernel_fpu_begin()

From: Shuah Khan
Date: Tue Jun 02 2020 - 13:27:13 EST

On 6/2/20 11:03 AM, Andy Lutomirski wrote:
On Tue, Jun 2, 2020 at 3:56 AM Borislav Petkov <bp@xxxxxxxxx> wrote:


On Tue, Jun 02, 2020 at 01:29:51PM +0300, Petteri Aimonen wrote:
The kernel module is not actually x86-specific, even though it is
currently only enabled for x86. amdgpu driver already does kernel mode
floating point operations on PPC64 also, and the same module could be
used to test the same thing there.

Then make it generic please and put the user portion in, say,
tools/testing/selftests/fpu/ and we can ask ppc people to test it too.
People might wanna add more stuff to it in the future, which would be

To deterministically trigger the bug, the syscall has to come from the
same thread that has modified MXCSR. Going through /usr/sbin/modprobe
won't work, and manually doing the necessary syscalls for module loading
seems too complicated.

Ok, fair enough. But put that file in debugfs pls.

I think I agree. While it would be delightful to have general
selftest tooling for kernel modules, we don't have that right now, and
having the test just work with an appropriately configured kernel
would be nice.

Let's extend it to do what we want it to do. I will happy to take
patches. If you have some concrete ideas on what we can add, please
do a short summary of what is missing. I will find a way to get this

How about putting the file you frob in
/sys/kernel/debug/selftest_helpers/something_or_other. The idea would
be that /sys/kernel/debug/selftest_helpers would be a general place
for kernel helpers needed to make selftests work.

Is this a workaround for the lack of selftest tooling for kernel
modules? In which case, let's us focus on fix selftest tooling.

-- Shuah