Re: [PATCH 1/6] arm64/sme: Flush foreign register state in do_sme_acc()

[Dave Martin]

On Tue, Dec 03, 2024 at 12:45:53PM +0000, Mark Brown wrote:
> When do_sme_acc() runs with foreign FP state it does not do any updates of
> the task structure, relying on the next return to userspace to reload the
> register state appropriately, but leaves the task's last loaded CPU
> untouched. This means that if the task returns to userspace on the last
> CPU it ran on then the checks in fpsimd_bind_task_to_cpu() will incorrectly
> determine that the register state on the CPU is current and suppress reload
> of the floating point register state before returning to userspace. This
> will result in spurious warnings due to SME access traps occuring for the
> task after TIF_SME is set.
> 
> Call fpsimd_flush_task_state() to invalidate the last loaded CPU
> recorded in the task, forcing detection of the task as foreign.
> 
> Fixes: 8bd7f91c03d8 ("arm64/sme: Implement traps and syscall handling for SME")
> Reported-by: Mark Rutlamd <mark.rutland@xxxxxxx>
> Signed-off-by: Mark Brown <broonie@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> ---
>  arch/arm64/kernel/fpsimd.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
> index 8c4c1a2186cc510a7826d15ec36225857c07ed71..eca0b6a2fc6fa25d8c850a5b9e109b4d58809f54 100644
> --- a/arch/arm64/kernel/fpsimd.c
> +++ b/arch/arm64/kernel/fpsimd.c
> @@ -1460,6 +1460,8 @@ void do_sme_acc(unsigned long esr, struct pt_regs *regs)
>  		sme_set_vq(vq_minus_one);
>  
>  		fpsimd_bind_task_to_cpu();
> +	} else {
> +		fpsimd_flush_task_state(current);

TIF_FOREIGN_FPSTATE is (or was) a cache of the task<->CPU binding that
you're clobbering here.

So, this fpsimd_flush_task_state() should have no effect unless
TIF_FOREIGN_FPSTATE is already wrong?  I'm wondering if the apparent
need for this means that there is an undiagnosed bug elsewhere.

(My understanding is based on FPSIMD/SVE; I'm less familiar with the
SME changes, so I may be missing something important here.)

[...]

Cheers
---Dave