Re: [PATCH 2/3] target/arm: Set KVM_ARM_VCPU_SVE while probing the host

[Richard Henderson]

On 7/25/22 11:14, Richard Henderson wrote:
> Because we weren't setting this flag, our probe of ID_AA64ZFR0
> was always returning zero.  This also obviates the adjustment
> of ID_AA64PFR0, which had sanitized the SVE field.


Oh, I meant to say here that this the effects of the bug are not visible, because the only 
thing that ISAR.ID_AA64ZFR0 is used for within qemu at present is tcg translation.  The 
other tests for SVE within KVM are via ISAR.ID_AA64PFR0.SVE.


r~


> Reported-by: Zenghui Yu <yuzenghui@huawei.com>
> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
> ---
>   target/arm/kvm64.c | 27 +++++++++++++--------------
>   1 file changed, 13 insertions(+), 14 deletions(-)
>
> diff --git a/target/arm/kvm64.c b/target/arm/kvm64.c
> index 36ff20c9e3..8b2ae9948a 100644
> --- a/target/arm/kvm64.c
> +++ b/target/arm/kvm64.c
> @@ -512,7 +512,6 @@ bool kvm_arm_get_host_cpu_features(ARMHostCPUFeatures *ahcf)
>       bool sve_supported;
>       bool pmu_supported = false;
>       uint64_t features = 0;
> -    uint64_t t;
>       int err;
>   
>       /* Old kernels may not know about the PREFERRED_TARGET ioctl: however
> @@ -533,10 +532,17 @@ bool kvm_arm_get_host_cpu_features(ARMHostCPUFeatures 
> *ahcf)
>       struct kvm_vcpu_init init = { .target = -1, };
>   
>       /*
> -     * Ask for Pointer Authentication if supported. We can't play the
> -     * SVE trick of synthesising the ID reg as KVM won't tell us
> -     * whether we have the architected or IMPDEF version of PAuth, so
> -     * we have to use the actual ID regs.
> +     * Ask for SVE if supported, so that we can query ID_AA64ZFR0,
> +     * which is otherwise RAZ.
> +     */
> +    sve_supported = kvm_arm_svm_supported();
> +    if (sve_supported) {
> +        init.features[0] |= 1 << KVM_ARM_VCPU_SVE;
> +    }
> +
> +    /*
> +     * Ask for Pointer Authentication if supported, so that we get
> +     * the unsanitized field values for AA64ISAR1_EL1.
>        */
>       if (kvm_arm_pauth_supported()) {
>           init.features[0] |= (1 << KVM_ARM_VCPU_PTRAUTH_ADDRESS |
> @@ -680,20 +686,13 @@ bool kvm_arm_get_host_cpu_features(ARMHostCPUFeatures 
> *ahcf)
>           }
>       }
>   
> -    sve_supported = kvm_arm_svm_supported();
> -
> -    /* Add feature bits that can't appear until after VCPU init. */
>       if (sve_supported) {
> -        t = ahcf->isar.id_aa64pfr0;
> -        t = FIELD_DP64(t, ID_AA64PFR0, SVE, 1);
> -        ahcf->isar.id_aa64pfr0 = t;
> -
>           /*
>            * There is a range of kernels between kernel commit 73433762fcae
>            * and f81cb2c3ad41 which have a bug where the kernel doesn't expose
>            * SYS_ID_AA64ZFR0_EL1 via the ONE_REG API unless the VM has enabled
> -         * SVE support, so we only read it here, rather than together with all
> -         * the other ID registers earlier.
> +         * SVE support, which resulted in an error rather than RAZ.
> +         * So only read the register if we set KVM_ARM_VCPU_SVE above.
>            */
>           err |= read_sys_reg64(fdarray[2], &ahcf->isar.id_aa64zfr0,
>                                 ARM64_SYS_REG(3, 0, 0, 4, 4));