Re: [PATCH for-8.2] target/arm: Disable SME if SVE is disabled

[Cornelia Huck <cohuck@redhat.com>]

On Mon, Nov 27 2023, Peter Maydell <peter.maydell@linaro.org> wrote:

> There is no architectural requirement that SME implies SVE, but
> our implementation currently assumes it. (FEAT_SME_FA64 does
> imply SVE.) So if you try to run a CPU with eg "-cpu max,sve=off"
> you quickly run into an assert when the guest tries to write to
> SMCR_EL1:
>
> #6  0x00007ffff4b38e96 in __GI___assert_fail
>     (assertion=0x5555566e69cb "sm", file=0x5555566e5b24 "../../target/arm/helper.c", line=6865, function=0x5555566e82f0 <__PRETTY_FUNCTION__.31> "sve_vqm1_for_el_sm") at ./assert/assert.c:101
> #7  0x0000555555ee33aa in sve_vqm1_for_el_sm (env=0x555557d291f0, el=2, sm=false) at ../../target/arm/helper.c:6865
> #8  0x0000555555ee3407 in sve_vqm1_for_el (env=0x555557d291f0, el=2) at ../../target/arm/helper.c:6871
> #9  0x0000555555ee3724 in smcr_write (env=0x555557d291f0, ri=0x555557da23b0, value=2147483663) at ../../target/arm/helper.c:6995
> #10 0x0000555555fd1dba in helper_set_cp_reg64 (env=0x555557d291f0, rip=0x555557da23b0, value=2147483663) at ../../target/arm/tcg/op_helper.c:839
> #11 0x00007fff60056781 in code_gen_buffer ()
>
> Avoid this unsupported and slightly odd combination by
> disabling SME when SVE is not present.
>
> Cc: qemu-stable@nongnu.org
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2005
> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
> ---
> '-cpu sve=off,sme=on,sme_fa64=off' crashes in the same way, so just
> turning off FA64 isn't sufficient.  Maybe we should support
> SME-no-SVE, but for 8.2 at least turning off SME is better than
> letting users hit an assertion.
> ---
>  target/arm/cpu.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
>
> diff --git a/target/arm/cpu.c b/target/arm/cpu.c
> index 25e9d2ae7b8..0fe268ac785 100644
> --- a/target/arm/cpu.c
> +++ b/target/arm/cpu.c
> @@ -1743,6 +1743,15 @@ void arm_cpu_finalize_features(ARMCPU *cpu, Error **errp)
>              return;
>          }
>  
> +        /*
> +         * FEAT_SME is not architecturally dependent on FEAT_SVE (unless
> +         * FEAT_SME_FA64 is present). However our implementation currently
> +         * assumes it, so if the user asked for sve=off then turn off SME also.
> +         */

Might be worth adding a note here that KVM currently does not support
SME anyway? It took me a moment to remember that.

> +        if (cpu_isar_feature(aa64_sme, cpu) && !cpu_isar_feature(aa64_sve, cpu)) {
> +            object_property_set_bool(OBJECT(cpu), "sme", false, &error_abort);
> +        }
> +
>          arm_cpu_sme_finalize(cpu, &local_err);
>          if (local_err != NULL) {
>              error_propagate(errp, local_err);