Re: [PATCH v7 27/27] KVM: arm64/sve: Document KVM API extensions for SVE

Dave Martin
2019-04-05T09:37:48Z

On Thu, Apr 04, 2019 at 11:09:21PM +0200, Andrew Jones wrote:
> On Fri, Mar 29, 2019 at 01:00:52PM +0000, Dave Martin wrote:
> > 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>
> >
> > ---
> >
> > Changes since v5:
> >
> > * Document KVM_ARM_VCPU_FINALIZE and its interactions with SVE.
> > ---
> > Documentation/virtual/kvm/api.txt | 132 +++++++++++++++++++++++++++++++++++++-
> > 1 file changed, 129 insertions(+), 3 deletions(-)
> >
> > diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
> > index cd920dd..68509de 100644
> > --- a/Documentation/virtual/kvm/api.txt
> > +++ b/Documentation/virtual/kvm/api.txt
> > @@ -1873,6 +1873,7 @@ Parameters: struct kvm_one_reg (in)
> > Returns: 0 on success, negative value on failure
> > Errors:
> >  ENOENT:   no such register
> > +  EPERM:    register access forbidden for architecture-dependent reasons
> >  EINVAL:   other errors, such as bad size encoding for a known register
> >
> > struct kvm_one_reg {
> > @@ -2127,13 +2128,20 @@ 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_VCPU_INIT.
> > +
> > + The equivalent register content can be accessed via bits [127:0] of
> > + 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>
> >
> > @@ -2143,6 +2151,61 @@ 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]
> > + 0x6060 0000 0015 ffff KVM_REG_ARM64_SVE_VLS pseudo-register
> > +
> > +Access to slices beyond the maximum vector length configured for the
> > +vcpu (i.e., where 16 * slice >= max_vq (**)) will fail with ENOENT.
>
> I've been doing pretty well keeping track of the 16's, 128's, VL's and
> VQ's, but this example lost me. Also, should it be >= or just > ?

It should be >=. It's analogous to not being allowed to derefence an
array index that is >= the size of the array.

Also, the 16 here is not the number of bytes per quadword (as it often
does with all things SVE), but the number of quadwords per 2048-bit
KVM register slice.

To make matters worse (**) resembles some weird C syntax.

Maybe this would be less confusing written as

Access to register IDs where 2048 * slice >= 128 * max_vq will fail
with ENOENT. max_vq is the vcpu's maximum supported vector length
in 128-bit quadwords: see (**) below.

Does that help at all?

>
> > +
> > +These registers are only accessible on vcpus for which SVE is enabled.
> > +See KVM_ARM_VCPU_INIT for details.
> > +
> > +In addition, except for KVM_REG_ARM64_SVE_VLS, these registers are not
> > +accessible until the vcpu's SVE configuration has been finalized
> > +using KVM_ARM_VCPU_FINALIZE(KVM_ARM_VCPU_SVE). See KVM_ARM_VCPU_INIT
> > +and KVM_ARM_VCPU_FINALIZE for more information about this procedure.
> > +
> > +KVM_REG_ARM64_SVE_VLS is a pseudo-register that allows the set of vector
> > +lengths supported by the vcpu to be discovered and configured by
> > +userspace. When transferred to or from user memory via KVM_GET_ONE_REG
> > +or KVM_SET_ONE_REG, the value of this register is of type __u64[8], and
> > +encodes the set of vector lengths as follows:
> > +
> > +__u64 vector_lengths[8];
> > +
> > +if (vq >= SVE_VQ_MIN && vq <= SVE_VQ_MAX &&
> > + ((vector_lengths[(vq - 1) / 64] >> ((vq - 1) % 64)) & 1))
> > + /* Vector length vq * 16 bytes supported */
> > +else
> > + /* Vector length vq * 16 bytes not supported */
> > +
> > +(**) 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.
> > +
> > +(See Documentation/arm64/sve.txt for an explanation of the "vq"
> > +nomenclature.)
> > +
> > +KVM_REG_ARM64_SVE_VLS is only accessible after KVM_ARM_VCPU_INIT.
> > +KVM_ARM_VCPU_INIT initialises it to the best set of vector lengths that
> > +the host supports.
> > +
> > +Userspace may subsequently modify it if desired until the vcpu's SVE
> > +configuration is finalized using KVM_ARM_VCPU_FINALIZE(KVM_ARM_VCPU_SVE).
> > +
> > +Apart from simply removing all vector lengths from the host set that
> > +exceed some value, support for arbitrarily chosen sets of vector lengths
> > +is hardware-dependent and may not be available. Attempting to configure
> > +an invalid set of vector lengths via KVM_SET_ONE_REG will fail with
> > +EINVAL.
> > +
> > +After the vcpu's SVE configuration is finalized, further attempts to
> > +write this register will fail with EPERM.
> > +
> >
> > MIPS registers are mapped using the lower 32 bits. The upper 16 of that is
> > the register group type:
> > @@ -2197,6 +2260,7 @@ Parameters: struct kvm_one_reg (in and out)
> > Returns: 0 on success, negative value on failure
> > Errors:
> >  ENOENT:   no such register
> > +  EPERM:    register access forbidden for architecture-dependent reasons
> >  EINVAL:   other errors, such as bad size encoding for a known register
> >
> > This ioctl allows to receive the value of a single register implemented
> > @@ -2690,6 +2754,33 @@ Possible features:
> > - KVM_ARM_VCPU_PMU_V3: Emulate PMUv3 for the CPU.
> > Depends on KVM_CAP_ARM_PMU_V3.
> >
> > + - KVM_ARM_VCPU_SVE: Enables SVE for the CPU (arm64 only).
> > + Depends on KVM_CAP_ARM_SVE.
> > + Requires KVM_ARM_VCPU_FINALIZE(KVM_ARM_VCPU_SVE):
> > +
> > + * After KVM_ARM_VCPU_INIT:
> > +
> > + - KVM_REG_ARM64_SVE_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_SVE):
> > +
> > + - 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 archietctural SVE registers
> > + KVM_REG_ARM64_SVE_ZREG(), KVM_REG_ARM64_SVE_PREG() or
> > + KVM_REG_ARM64_SVE_FFR;
> > +
> > + - KVM_REG_ARM64_SVE_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_SVE):
> > +
> > + - the KVM_REG_ARM64_SVE_VLS pseudo-register is immutable, and can
> > + no longer be written using KVM_SET_ONE_REG.
> >
> > 4.83 KVM_ARM_PREFERRED_TARGET
> >
> > @@ -3904,6 +3995,41 @@ number of valid entries in the 'entries' array, which is then filled.
> > 'index' and 'flags' fields in 'struct kvm_cpuid_entry2' are currently reserved,
> > userspace should not expect to get any particular value there.
> >
> > +4.119 KVM_ARM_VCPU_FINALIZE
> > +
> > +Capability: KVM_CAP_ARM_SVE
>
> The KVM_ARM_VCPU_FINALIZE vcpu ioctl isn't SVE specific, so it shouldn't
> depend on the SVE cap. It should have it's own cap, or maybe it should
> just be an KVM_ARM_VCPU_SVE_FINALIZE ioctl instead, i.e. not general.

This one's a bit annoying. This would ideally read

Capability: KVM_CAP_ARM_SVE, or any other capability that advertises the
availability of a feature that requires KVM_ARM_VCPU_FINALIZE to be used.

(which sounds vague).

We could add a specific cap for KVM_ARM_VCPU_FINALIZE, but that seemed
overkill. Or document KVM_ARM_VCPU_FINALIZE as unconditionally
supported, but make the individual feature values cap-dependent. This
works because the symptom of missing support is the same (EINVAL)
ragardless of whether it is the specific feature or
KVM_ARM_VCPU_FINALIZE that is unsupported.

Thoughts?

> > +Architectures: arm, arm64
> > +Type: vcpu ioctl
> > +Parameters: int feature (in)
>
> This was called 'what' in the code.

Indeed it is. I wanted to avoid the implication that this paramter
exactly maps onto the KVM_ARM_VCPU_INIT features. But "what" seemed
a bit too vague for the documentation.

Mind you, if lseek() can have a "whence" parameter, perhaps "what" is
also acceptable.

OTOH I don't mind changing the name in the code to "feature" if you
think that's preferable.

Thoughts?

>
> > +Returns: 0 on success, -1 on error
> > +Errors:
> > + EPERM: feature not enabled, needs configuration, or already finalized
> > + EINVAL: unknown feature
> > +
> > +Recognised values for feature:
> > + arm64 KVM_ARM_VCPU_SVE
> > +
> > +Finalizes the configuration of the specified vcpu feature.
> > +
> > +The vcpu must already have been initialised, enabling the affected feature, by
> > +means of a successful KVM_ARM_VCPU_INIT call with the appropriate flag set in
> > +features[].
> > +
> > +For affected vcpu features, this is a mandatory step that must be performed
> > +before the vcpu is fully usable.
> > +
> > +Between KVM_ARM_VCPU_INIT and KVM_ARM_VCPU_FINALIZE, the feature may be
> > +configured by use of ioctls such as KVM_SET_ONE_REG. The exact configuration
> > +that should be performaned and how to do it are feature-dependent.
> > +
> > +Other calls that depend on a particular feature being finalized, such as
> > +KVM_RUN, KVM_GET_REG_LIST, KVM_GET_ONE_REG and KVM_SET_ONE_REG, will fail with
> > +-EPERM unless the feature has already been finalized by means of a
> > +KVM_ARM_VCPU_FINALIZE call.
> > +
> > +See KVM_ARM_VCPU_INIT for details of vcpu features that require finalization
> > +using this ioctl.
> > +
> > 5. The kvm_run structure
> > ------------------------
> >
>
> I'm still not sure about EPERM vs. ENOEXEC.

There is no need to distinguish the two: this is just a generic "vcpu in
wrong state for this to work" error. I can't think of another subsystem
that uses ENOEXEC for this meaning, but it's established in KVM.

If you can't see a reason why we would need these to be distinct
errors (?) then I'm happy for this to be ENOEXEC for all cases.

Cheers
---Dave