On 8/21/25 6:53 PM, Mark Brown wrote:
> SME, the Scalable Matrix Extension, is an arm64 extension which adds
> support for matrix operations, with core concepts patterned after SVE.
>
> SVE introduced some complication in the ABI since it adds new vector
> floating point registers with runtime configurable size, the size being
> controlled by a prameter called the vector length (VL). To provide control
parameter
> of this to VMMs we offer two phase configuration of SVE, SVE must first be
> enabled for the vCPU with KVM_ARM_VCPU_INIT(KVM_ARM_VCPU_SVE), after which
> vector length may then be configured but the configurably sized floating
> point registers are inaccessible until finalized with a call to
> KVM_ARM_VCPU_FINALIZE(KVM_ARM_VCPU_SVE) after which the configurably sized
> registers can be accessed.
>
...
>
> Signed-off-by: Mark Brown <broonie@xxxxxxxxxx>
> ---
> Documentation/virt/kvm/api.rst | 117 +++++++++++++++++++++++++++++------------
> 1 file changed, 82 insertions(+), 35 deletions(-)
>
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index 6aa40ee05a4a..71f46b342641 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -2600,12 +2600,12 @@ Specifically:
> 0x6020 0000 0010 00d5 FPCR 32 fp_regs.fpcr
> ======================= ========= ===== =======================================
>
> -.. [1] These encodings are not accepted for SVE-enabled vcpus. See
> - :ref:`KVM_ARM_VCPU_INIT`.
> +.. [1] These encodings are not accepted for SVE enabled vcpus. See
SVE-enabled
was good.
> + :ref:`KVM_ARM_VCPU_INIT`. They are also not accepted when SME is
> + enabled without SVE and the vcpu is in streaming mode.
>
> @@ -2665,19 +2675,25 @@ follows::
> /* Vector length vq * 16 bytes not supported */
>
> .. [2] The maximum value vq for which the above condition is true is
> - max_vq. This is the maximum vector length available to the guest on
> - this vcpu, and determines which register slices are visible through
> - this ioctl interface.
> + max_vq. This is the maximum vector length currently available to
> + the guest on this vcpu, and determines which register slices are
> + visible through this ioctl interface.
> +
> + If SME is supported then the max_vq used for the Z and P registers
> + then while SVCR.SM is 1 this vector length will be the maximum SME
I'm having trouble parsing the 2 lines above.
> + vector length available for the guest, otherwise it will be the
> + maximum SVE vector length available.
>
> (See Documentation/arch/arm64/sve.rst for an explanation of the "vq"
> nomenclature.)
>
> @@ -3520,7 +3537,7 @@ Possible features:
> initial value of this pseudo-register indicates the best set of
> vector lengths possible for a vcpu on this host.
>
> - * Before KVM_ARM_VCPU_FINALIZE(KVM_ARM_VCPU_SVE):
> + * Before KVM_ARM_VCPU_FINALIZE(KVM_ARM_VCPU_VEC}):
Why the inserted '}', please?
>
> - KVM_RUN and KVM_GET_REG_LIST are not available;
>
> @@ -3533,11 +3550,40 @@ Possible features:
> KVM_SET_ONE_REG, to modify the set of vector lengths available
> for the vcpu.
>
> - * After KVM_ARM_VCPU_FINALIZE(KVM_ARM_VCPU_SVE):
> + * After KVM_ARM_VCPU_FINALIZE(KVM_ARM_VCPU_VEC):
>
> - the KVM_REG_ARM64_SVE_VLS pseudo-register is immutable, and can
> no longer be written using KVM_SET_ONE_REG.
>
> + - KVM_ARM_VCPU_SME: Enables SME for the CPU (arm64 only).
> + Depends on KVM_CAP_ARM_SME.
> + Requires KVM_ARM_VCPU_FINALIZE(KVM_ARM_VCPU_VEC):
> +
> + * After KVM_ARM_VCPU_INIT:
> +
> + - KVM_REG_ARM64_SME_VLS may be read using KVM_GET_ONE_REG: the
> + initial value of this pseudo-register indicates the best set of
> + vector lengths possible for a vcpu on this host.
> +
> + * Before KVM_ARM_VCPU_FINALIZE(KVM_ARM_VCPU_VEC}):
ditto: inserted '}'
> +
> + - KVM_RUN and KVM_GET_REG_LIST are not available;
> +
> + - KVM_GET_ONE_REG and KVM_SET_ONE_REG cannot be used to access
> + the scalable architectural SVE registers
> + KVM_REG_ARM64_SVE_ZREG(), KVM_REG_ARM64_SVE_PREG() or
> + KVM_REG_ARM64_SVE_FFR, the matrix register
> + KVM_REG_ARM64_SME_ZA() or the LUT register KVM_REG_ARM64_ZT();
> +
> + - KVM_REG_ARM64_SME_VLS may optionally be written using
> + KVM_SET_ONE_REG, to modify the set of vector lengths available
> + for the vcpu.
> +
> + * After KVM_ARM_VCPU_FINALIZE(KVM_ARM_VCPU_VEC):
--
~Randy
