Re: [kvm-unit-tests PATCH v5] arm: Replace MAX_SMP probe loop in favor of reading directly

[Andrew Jones <andrew.jones@linux.dev>]

On Tue, Feb 07, 2023 at 11:32:56PM +0000, Colton Lewis wrote:
> Replace the MAX_SMP probe loop in favor of reading a number directly
> from the QEMU error message. This is equally safe as the existing code
> because the error message has had the same format as long as it has
> existed, since QEMU v2.10. The final number before the end of the
> error message line indicates the max QEMU supports.
> 
> This loop logic is broken for machines with a number of CPUs that
> isn't a power of two. This problem was noticed for gicv2 tests on
> machines with a non-power-of-two number of CPUs greater than 8 because
> tests were running with MAX_SMP less than 8. As a hypothetical example,
> a machine with 12 CPUs will test with MAX_SMP=6 because 12 >> 1 ==
> 6. This can, in rare circumstances, lead to different test results
> depending only on the number of CPUs the machine has.
> 
> A previous comment explains the loop should only apply to kernels
> <=v4.3 on arm and suggests deletion when it becomes tiresome to
> maintain. However, it is always theoretically possible to test on a
> machine that has more CPUs than QEMU supports, so it makes sense to
> leave some check in place.
> 
> Signed-off-by: Colton Lewis <coltonlewis@google.com>
> ---
>  scripts/runtime.bash | 17 ++++++++---------
>  1 file changed, 8 insertions(+), 9 deletions(-)
> 
> v5: Remove the last awk reference and guard the probing code with a
> check that ARCH = arm or arm64.
> 
> v4: https://lore.kernel.org/kvm/20230201172110.1970980-1-coltonlewis@google.com/
> 
> v3: https://lore.kernel.org/kvm/20230130195700.729498-1-coltonlewis@google.com/
> 
> v2: https://lore.kernel.org/kvm/20230111215422.2153645-1-coltonlewis@google.com/
> 
> v1: https://lore.kernel.org/kvm/20221219185250.631503-1-coltonlewis@google.com/
> 
> diff --git a/scripts/runtime.bash b/scripts/runtime.bash
> index f8794e9a..fb64e855 100644
> --- a/scripts/runtime.bash
> +++ b/scripts/runtime.bash
> @@ -188,12 +188,11 @@ function run()
>  # Probe for MAX_SMP, in case it's less than the number of host cpus.
>  #
>  # This probing currently only works for ARM, as x86 bails on another
> -# error first. Also, this probing isn't necessary for any ARM hosts
> -# running kernels later than v4.3, i.e. those including ef748917b52
> -# "arm/arm64: KVM: Remove 'config KVM_ARM_MAX_VCPUS'". So, at some
> -# point when maintaining the while loop gets too tiresome, we can
> -# just remove it...
> -while $RUNTIME_arch_run _NO_FILE_4Uhere_ -smp $MAX_SMP \
> -		|& grep -qi 'exceeds max CPUs'; do
> -	MAX_SMP=$((MAX_SMP >> 1))
> -done
> +# error first, so this check is only run for ARM and ARM64. The
> +# parameter expansion takes the last number from the QEMU error
> +# message, which gives the allowable MAX_SMP.
> +if [ "${ARCH%64}" = arm ] && smp=$($RUNTIME_arch_run _NO_FILE_4Uhere_ -smp $MAX_SMP \
> +      |& grep 'exceeds max CPUs'); then
> +	smp=${smp##*(}
> +	MAX_SMP=${smp:0:-1}
> +fi
> --
> 2.39.1.519.gcb327c4b5f-goog

Applied, thanks