Re: [PATCH] arm64: Restore forced disabling of KPTI on ThunderX

[Suzuki K Poulose <suzuki.poulose@arm.com>]

On 22/09/2021 14:59, dann frazier wrote:
> A noted side-effect of commit 0c6c2d3615ef ("arm64: Generate cpucaps.h")
> is that cpucaps are now sorted, changing the enumeration order. This
> assumed no dependencies between cpucaps, which turned out not to be true
> in one case. UNMAP_KERNEL_AT_EL0 currently needs to be processed after
> WORKAROUND_CAVIUM_27456. ThunderX systems are incompatible with KPTI, so
> unmap_kernel_at_el0() bails if WORKAROUND_CAVIUM_27456 is set. But because
> of the sorting, WORKAROUND_CAVIUM_27456 will not yet have been considered
> when unmap_kernel_at_el0() checks for it, so the kernel tries to
> run w/ KPTI - and quickly falls over.
> 
> Because all ThunderX implementations have homogeneous CPUs, we can remove
> this dependency by just checking the current CPU for the erratum.
> 
> Fixes: 0c6c2d3615ef ("arm64: Generate cpucaps.h")
> Suggested-by: Suzuki K Poulose <suzuki.poulose@arm.com>
> Cc: stable@vger.kernel.org # 5.13+
> Signed-off-by: dann frazier <dann.frazier@canonical.com>
> ---
>   arch/arm64/kernel/cpufeature.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index f8a3067d10c6..7275b49034f3 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -1528,7 +1528,7 @@ static bool unmap_kernel_at_el0(const struct arm64_cpu_capabilities *entry,
>   	 * ThunderX leads to apparent I-cache corruption of kernel text, which
>   	 * ends as well as you might imagine. Don't even try.
>   	 */
> -	if (cpus_have_const_cap(ARM64_WORKAROUND_CAVIUM_27456)) {
> +	if (this_cpu_has_cap(ARM64_WORKAROUND_CAVIUM_27456)) {

Please could you also update the comment right above this line to
explain, why we do this and why this is fine (just like you have
in the description) ? Something like :

	 * Since we cannot rely on the order of the cpucaps
  	 * we cannot rely on the cpus_have_*cap() helpers to
	 * detect the erratum on the system. However, since
	 * affected CPUs are always in a homoegeneous configuration
	 * we could rely on this_cpu_has_cap()
	 */

So that looking at the code, it is easier to comprehend what we
figured out in the mailing list (and the description)

With that:

Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>

Suzuki

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