Re: [RFC PATCH v2 23/23] KVM: arm64/sve: Document KVM API extensions for SVE

["Alex Bennée" <alex.bennee@linaro.org>]

Dave Martin <Dave.Martin@arm.com> writes:

> This patch adds sections to the KVM API documentation describing
> the extensions for supporting the Scalable Vector Extension (SVE)
> in guests.
>
> Signed-off-by: Dave Martin <Dave.Martin@arm.com>
> ---
>  Documentation/virtual/kvm/api.txt | 142 +++++++++++++++++++++++++++++++++++++-
>  1 file changed, 139 insertions(+), 3 deletions(-)
>
> diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
> index a58067b..b8257d4 100644
> --- a/Documentation/virtual/kvm/api.txt
> +++ b/Documentation/virtual/kvm/api.txt
> @@ -2054,13 +2054,21 @@ Specifically:
>    0x6030 0000 0010 004c SPSR_UND    64  spsr[KVM_SPSR_UND]
>    0x6030 0000 0010 004e SPSR_IRQ    64  spsr[KVM_SPSR_IRQ]
>    0x6060 0000 0010 0050 SPSR_FIQ    64  spsr[KVM_SPSR_FIQ]
> -  0x6040 0000 0010 0054 V0         128  fp_regs.vregs[0]
> -  0x6040 0000 0010 0058 V1         128  fp_regs.vregs[1]
> +  0x6040 0000 0010 0054 V0         128  fp_regs.vregs[0]    (*)
> +  0x6040 0000 0010 0058 V1         128  fp_regs.vregs[1]    (*)
>      ...
> -  0x6040 0000 0010 00d0 V31        128  fp_regs.vregs[31]
> +  0x6040 0000 0010 00d0 V31        128  fp_regs.vregs[31]   (*)
>    0x6020 0000 0010 00d4 FPSR        32  fp_regs.fpsr
>    0x6020 0000 0010 00d5 FPCR        32  fp_regs.fpcr
>
> +(*) These encodings are not accepted for SVE-enabled vcpus.  See
> +    KVM_ARM_SVE_CONFIG for details of how SVE support is configured for
> +    a vcpu.
> +
> +    The equivalent register content can be accessed via bits [2047:0]
> of

You mean [127:0] I think.

> +    the corresponding SVE Zn registers instead for vcpus that have SVE
> +    enabled (see below).
> +
>  arm64 CCSIDR registers are demultiplexed by CSSELR value:
>    0x6020 0000 0011 00 <csselr:8>
>
> @@ -2070,6 +2078,14 @@ arm64 system registers have the following id bit patterns:
>  arm64 firmware pseudo-registers have the following bit pattern:
>    0x6030 0000 0014 <regno:16>
>
> +arm64 SVE registers have the following bit patterns:
> +  0x6080 0000 0015 00 <n:5> <slice:5>   Zn bits[2048*slice + 2047 : 2048*slice]
> +  0x6050 0000 0015 04 <n:4> <slice:5>   Pn bits[256*slice + 255 : 256*slice]
> +  0x6050 0000 0015 060 <slice:5>        FFR bits[256*slice + 255 : 256*slice]
> +
> +  These registers are only accessible on SVE-enabled vcpus.  See
> +  KVM_ARM_SVE_CONFIG for details.
> +
>
>  MIPS registers are mapped using the lower 32 bits.  The upper 16 of that is
>  the register group type:
> @@ -3700,6 +3716,126 @@ Returns: 0 on success, -1 on error
>  This copies the vcpu's kvm_nested_state struct from userspace to the kernel.  For
>  the definition of struct kvm_nested_state, see KVM_GET_NESTED_STATE.
>
> +4.116 KVM_ARM_SVE_CONFIG
> +
> +Capability: KVM_CAP_ARM_SVE
> +Architectures: arm64
> +Type: vm and vcpu ioctl
> +Parameters: struct kvm_sve_vls (in/out)
> +Returns: 0 on success
> +Errors:
> +  EINVAL:    Unrecognised subcommand or bad arguments
> +  EBADFD:    vcpu in wrong state for request
> +             (KVM_ARM_SVE_CONFIG_SET, KVM_ARM_SVE_CONFIG_SET)
> +  ENOMEM:    Out of memory
> +  EFAULT:    Bad user address
> +
> +struct kvm_sve_vls {
> +	__u16 cmd;
> +	__u16 max_vq;
> +	__u16 _reserved[2];
> +	__u64 required_vqs[8];
> +};
> +
> +General:
> +
> +cmd: This ioctl supports a few different subcommands, selected by the
> +value of cmd (described in detail in the following sections).
> +
> +_reserved[]: these fields may be meaningful to later kernels.  For
> +forward compatibility, they must be zeroed before invoking this ioctl
> +for the first time on a given struct kvm_sve_vls object.  (So, memset()
> +it to zero before first use, or allocate with calloc() for example.)
> +
> +max_vq, required_vqs[]: encode a set of SVE vector lengths.  The set is
> +encoded as follows:
> +
> +If (a * 64 + b + 1) <= max_vq, then the bit represented by
> +
> +    required_vqs[a] & ((__u64)1 << b)
> +
> +(where a is in the range 0..7 and b is in the range 0..63)
> +indicates that the vector length (a * 64 + b + 1) * 128 bits is
> +supported (KVM_ARM_SVE_CONFIG_QUERY, KVM_ARM_SVE_CONFIG_GET) or required
> +(KVM_ARM_SVE_CONFIG_SET).
> +
> +If (a * 64 + b + 1) > max_vq, then the vector length
> +(a * 64 + b + 1) * 128 bits is unsupported or prohibited respectively.
> +In other words, only the first max_vq bits in required_vqs[] are
> +significant; remaining bits are implicitly treated as if they were zero.
> +
> +max_vq must be in the range SVE_VQ_MIN (1) to SVE_VQ_MAX (512).
> +
> +See Documentation/arm64/sve.txt for an explanation of vector lengths and
> +the meaning associated with "VQ".
> +
> +Subcommands:
> +
> +/* values for cmd: */
> +#define KVM_ARM_SVE_CONFIG_QUERY	0 /* query what the host can support */
> +#define KVM_ARM_SVE_CONFIG_SET		1 /* enable SVE for vcpu and set VLs */
> +#define KVM_ARM_SVE_CONFIG_GET		2 /* read the set of VLs for a vcpu */
> +
> +Subcommand details:
> +
> +4.116.1 KVM_ARM_SVE_CONFIG_QUERY
> +Type: vm and vcpu
> +
> +Retrieve the full set of SVE vector lengths available for use by KVM
> +guests on this host.  The result is independent of which vcpu this
> +command is invoked on.  As a convenience, it may also be invoked on a
> +vm file descriptor, eliminating the need to create a vcpu first.
> +
> +4.116.2 KVM_ARM_SVE_CONFIG_SET
> +Type: vcpu only
> +
> +Enables SVE for the vcpu and sets the set of SVE vector lengths that
> +will be visible to the guest.
> +
> +This is the only way to enable SVE for a vcpu: if this command is not
> +invoked for a vcpu then SVE will not be available to the guest on this
> +vcpu.
> +
> +This subcommand is only permitted once per vcpu, before KVM_RUN has been
> +invoked for the vcpu for the first time.  Otherwise, the command fails
> +with -EBADFD and the state of the vcpu is not modified.
> +
> +In typical use, the user should call KVM_ARM_SVE_CONFIG_QUERY first to
> +populate a struct kvm_sve_vls with the full set of vector lengths
> +available on the host, then set cmd = KVM_ARM_SVE_CONFIG_SET and
> +re-issue the KVM_ARM_SVE_CONFIG ioctl on the desired vcpu.  This will
> +configure the best set of vector lengths available.  When following this
> +approach, the maximum available vector length can also be restricted by
> +reducing the value of max_vq before invoking KVM_ARM_SVE_CONFIG_SET.
> +
> +Every requested vector length in the struct kvm_sve_vls argument must be
> +supported by the hardware.  In addition, except for vector lengths
> +greater than the maximum requested vector length, every vector length
> +not requested must *not* be supported by the hardware.  (The latter
> +restriction may be relaxed in the future.)  If the requested set of
> +vector lengths is not supportable, the command fails with -EINVAL and
> +the state of the vcpu is not modified.
> +
> +Different vcpus of a vm may be configured with different sets of vector
> +lengths.  Equally, some vcpus may have SVE enabled and some not.
> +However, such configurations are not recommended except for testing and
> +experimentation purposes.  Architecturally compliant guest OSes will
> +work, but may or may not make effective use of the resulting
> +configuration.
> +
> +After a successful KVM_ARM_SVE_CONFIG_SET, KVM_ARM_SVE_CONFIG_GET can be
> +used to retrieve the configured set of vector lengths.
> +
> +4.116.3 KVM_ARM_SVE_CONFIG_GET
> +Type: vcpu only
> +
> +This subcommand returns the set of vector lengths enabled for the vcpu.
> +SVE must have been enabled and configured for this vcpu by a successful
> +prior KVM_ARM_SVE_CONFIG_SET call.  Otherwise, -EBADFD is returned.
> +
> +The state of the vcpu is unchanged.
> +
> +
>  5. The kvm_run structure
>  ------------------------


--
Alex Bennée
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