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SubjectRe: [PATCH v5 01/10] KVM: arm64: Document PV-time interface
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On 03/10/2019 13:19, Andrew Jones wrote:
> 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?

Fair point - I guess .rst is the way of the future!

>> @@ -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.

This is somewhat embarrassing. Apparently when I was reviewing the new
specification I didn't notice these (subtle) changes. Thanks for
pointing it out to me! I'll update the code to match.

>> +
>> +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.

I guess they are generally called "hypercalls" not "hyperfunctions" - so
I'll match the spec here.

>> + 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.

True - although the values easily fit in int32 too! But I'll update to
be consistent with 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".

Will update.

Thanks,

Steve

>> + 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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