target/arm: Disable SME if SVE is disabled

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

Posted by Peter Maydell 1 year ago

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.
+         */
+        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);
-- 
2.34.1

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

Posted by Richard Henderson 12 months ago

On 11/27/23 11:33, Peter Maydell 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.

My first reaction was simply to change smcr_write, so that it does not compute SVL unless 
SM (as otherwise the write does not (immediately) change vector length).

However, as I searched for other uses of sve_vqm1_for_el, I immediately ran into other 
places in which are gated solely by isar_feature_aa64_sve.  So I think this simple patch 
is best for 8.2.

I also slightly wonder if SME && !SVE is a useful combination.  AFAIK, while v9 does not 
*require* SVE, SVE2 is intended as a replacement for AdvSIMD, and I believe that so far 
all v9 cpus have at least 128-bit SVE.

In any case, I'll work on any improvements in this area for next cycle.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>


r~

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

Posted by Cornelia Huck 12 months ago

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);