On 5/8/24 17:29, Chang S. Bae wrote:
> +void kernel_fpu_reset(void)
> +{
> + kernel_fpu_begin();
> + if (cpu_feature_enabled(X86_FEATURE_AMX_TILE))
> + tile_release();
> + kernel_fpu_end();
> +}
> +EXPORT_SYMBOL(kernel_fpu_reset);
> +
...
> --- a/drivers/platform/x86/intel/ifs/runtest.c
> +++ b/drivers/platform/x86/intel/ifs/runtest.c
> @@ -188,6 +188,8 @@ static int doscan(void *data)
> /* Only the first logical CPU on a core reports result */
> first = cpumask_first(cpu_smt_mask(cpu));
>
> + kernel_fpu_reset();
> +
> wait_for_sibling_cpu(&scan_cpus_in, NSEC_PER_SEC);
Remember, kernel_fpu_begin/end() mark a section of code that needs the
FPU. Once code calls kernel_fpu_end(), it no longer owns the FPU and
all bets are off. A interrupt could theoretically come in and do
whatever it wants.
I _assume_ that this is practically impossible since the stop_machine()
infrastructure keeps interrupts at bay. But it's rather subtle.
I'd probably just do this:
+ kernel_fpu_begin();
+ // AMX *MUST* be in the init state for the wrmsr() to work.
+ // But, the more in the init state, the less state the test
+ // has to save and restore. Just zap everything.
+ restore_fpregs_from_fpstate(&init_fpstate,
+ fpu_user_cfg.max_features);
+
wrmsrl(MSR_ACTIVATE_SCAN, params->activate->data);
rdmsrl(MSR_SCAN_STATUS, status.data);
+ kernel_fpu_end();
That's dirt simple. It doesn't require new infrastructure. It doesn't
call an opaque new helper. It doesn't require a feature check. It
probably makes the IFS test run faster. It will also magically work for
any fancy new feature that comes along which *ALSO* needs to be in its
init state ... with zero changes to this code. For bonus points, this
code is quite universal. It will work, as-is, in a bunch of kernel
contexts if future deranged kernel developer copies and pastes it. The
code you suggested above can race unless it's called under
stop_machine() and isn't safe to copy elsewhere.
Three lines of code:
1. IFS declares its need to own the FPU for a moment, like any
other kernel_fpu_begin() user. It's not a special snowflake.
It is boring.
2. IFS zaps the FPU state
3. IFS gives up the FPU
Am I out of my mind? What am I missing? Why bother with _anything_
more complicated than this?
