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SubjectRe: [PATCH v5 01/10] KVM: arm64: Document PV-time interface
On Wed, Oct 02, 2019 at 03:50:28PM +0100, Steven Price wrote:
> Introduce a paravirtualization interface for KVM/arm64 based on the
> "Arm Paravirtualized Time for Arm-Base Systems" specification DEN 0057A.
>
> This only adds the details about "Stolen Time" as the details of "Live
> Physical Time" have not been fully agreed.
>
> User space can specify a reserved area of memory for the guest and
> inform KVM to populate the memory with information on time that the host
> kernel has stolen from the guest.
>
> A hypercall interface is provided for the guest to interrogate the
> hypervisor's support for this interface and the location of the shared
> memory structures.
>
> Signed-off-by: Steven Price <steven.price@arm.com>
> ---
> Documentation/virt/kvm/arm/pvtime.txt | 65 +++++++++++++++++++++++++
> Documentation/virt/kvm/devices/vcpu.txt | 14 ++++++
> 2 files changed, 79 insertions(+)
> create mode 100644 Documentation/virt/kvm/arm/pvtime.txt
>
> diff --git a/Documentation/virt/kvm/arm/pvtime.txt b/Documentation/virt/kvm/arm/pvtime.txt
> new file mode 100644
> index 000000000000..fa15c12eec91
> --- /dev/null
> +++ b/Documentation/virt/kvm/arm/pvtime.txt

Maybe use .rst instead of .txt?

> @@ -0,0 +1,65 @@
> +Paravirtualized time support for arm64
> +======================================
> +
> +Arm specification DEN0057/A defined a standard for paravirtualised time

s/defined/defines/ ?

> +support for AArch64 guests:
> +
> +https://developer.arm.com/docs/den0057/a
> +
> +KVM/arm64 implements the stolen time part of this specification by providing
> +some hypervisor service calls to support a paravirtualized guest obtaining a
> +view of the amount of time stolen from its execution.
> +
> +Two new SMCCC compatible hypercalls are defined:
> +
> +PV_FEATURES 0xC5000020

The spec calls this PV_TIME_FEATURES.

> +PV_TIME_ST 0xC5000022

This is 0xC5000021 in the spec.

> +
> +These are only available in the SMC64/HVC64 calling convention as
> +paravirtualized time is not available to 32 bit Arm guests. The existence of
> +the PV_FEATURES hypercall should be probed using the SMCCC 1.1 ARCH_FEATURES
> +mechanism before calling it.
> +
> +PV_FEATURES
> + Function ID: (uint32) : 0xC5000020
> + PV_func_id: (uint32) : The function to query for support.
> + Currently only PV_TIME_ST is supported.

The spec calls this PV_call_id, but maybe PV_func_id would have been better.

> + Return value: (int32) : NOT_SUPPORTED (-1) or SUCCESS (0) if the relevant
> + PV-time feature is supported by the hypervisor.

This is an int64 in the spec.

> +
> +PV_TIME_ST
> + Function ID: (uint32) : 0xC5000022
> + Return value: (int64) : IPA of the stolen time data structure for this
> + VCPU. On failure:
> + NOT_SUPPORTED (-1)
> +
> +The IPA returned by PV_TIME_ST should be mapped by the guest as normal memory
> +with inner and outer write back caching attributes, in the inner shareable
> +domain. A total of 16 bytes from the IPA returned are guaranteed to be
> +meaningfully filled by the hypervisor (see structure below).
> +
> +PV_TIME_ST returns the structure for the calling VCPU.
> +
> +Stolen Time
> +-----------
> +
> +The structure pointed to by the PV_TIME_ST hypercall is as follows:
> +
> + Field | Byte Length | Byte Offset | Description
> + ----------- | ----------- | ----------- | --------------------------
> + Revision | 4 | 0 | Must be 0 for version 0.1

The spec version is 1.0 and Table 1 says "For implementations compliant
with this revision of the specification...". So I think this description
should be "Must be 0 for version 1.0".

> + Attributes | 4 | 4 | Must be 0
> + Stolen time | 8 | 8 | Stolen time in unsigned
> + | | | nanoseconds indicating how
> + | | | much time this VCPU thread
> + | | | was involuntarily not
> + | | | running on a physical CPU.
> +
> +The structure will be updated by the hypervisor prior to scheduling a VCPU. It
> +will be present within a reserved region of the normal memory given to the
> +guest. The guest should not attempt to write into this memory. There is a
> +structure per VCPU of the guest.
> +
> +For the user space interface see Documentation/virt/kvm/devices/vcpu.txt
> +section "3. GROUP: KVM_ARM_VCPU_PVTIME_CTRL".
> +
> diff --git a/Documentation/virt/kvm/devices/vcpu.txt b/Documentation/virt/kvm/devices/vcpu.txt
> index 2b5dab16c4f2..6f3bd64a05b0 100644
> --- a/Documentation/virt/kvm/devices/vcpu.txt
> +++ b/Documentation/virt/kvm/devices/vcpu.txt
> @@ -60,3 +60,17 @@ time to use the number provided for a given timer, overwriting any previously
> configured values on other VCPUs. Userspace should configure the interrupt
> numbers on at least one VCPU after creating all VCPUs and before running any
> VCPUs.
> +
> +3. GROUP: KVM_ARM_VCPU_PVTIME_CTRL
> +Architectures: ARM64
> +
> +3.1 ATTRIBUTE: KVM_ARM_VCPU_PVTIME_IPA
> +Parameters: 64-bit base address
> +Returns: -ENXIO: Stolen time not implemented
> + -EEXIST: Base address already set for this VCPU
> + -EINVAL: Base address not 64 byte aligned
> +
> +Specifies the base address of the stolen time structure for this VCPU. The
> +base address must be 64 byte aligned and exist within a valid guest memory
> +region. See Documentation/virt/kvm/arm/pvtime.txt for more information
> +including the layout of the stolen time structure.
> --
> 2.20.1
>

Thanks,
drew

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