Subject: Re: [PATCH 1/5] KVM: arm64: Document PV-lock interface

From: Steven Price <[email-masked]>
Date: Tue, 17 Dec 2019 14:21:39 +0000

On Tue, Dec 17, 2019 at 01:55:45PM +0000, [email-masked] wrote:
> From: Zengruan Ye <[email-masked]>
> 
> Introduce a paravirtualization interface for KVM/arm64 to obtain the vcpu
> is currently running or not.
> 
> 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: Zengruan Ye <[email-masked]>
> ---
>  Documentation/virt/kvm/arm/pvlock.rst | 31 +++++++++++++++++++++++++++
>  1 file changed, 31 insertions(+)
>  create mode 100644 Documentation/virt/kvm/arm/pvlock.rst
> 
> diff --git a/Documentation/virt/kvm/arm/pvlock.rst b/Documentation/virt/kvm/arm/pvlock.rst
> new file mode 100644
> index 000000000000..eec0c36edf17
> --- /dev/null
> +++ b/Documentation/virt/kvm/arm/pvlock.rst
> @@ -0,0 +1,31 @@
> +.. SPDX-License-Identifier: GPL-2.0
> +
> +Paravirtualized lock support for arm64
> +======================================
> +
> +KVM/arm64 provids some hypervisor service calls to support a paravirtualized
> +guest obtaining the vcpu is currently running or not.
> +
> +Two new SMCCC compatible hypercalls are defined:
> +
> +* PV_LOCK_FEATURES:   0xC5000040
> +* PV_LOCK_PREEMPTED:  0xC5000041

These values are in the "Standard Hypervisor Service Calls" section of
SMCCC - so is there a document that describes this features such that
other OSes or hypervisors can implement it? I'm also not entirely sure
of the process of ensuring that the IDs picked are non-conflicting.

Otherwise if this is a KVM specific interface this should probably
belong within the "Vendor Specific Hypervisor Service Calls" section
along with some probing that the hypervisor is actually KVM. Although I
don't see anything KVM specific.

> +
> +The existence of the PV_LOCK hypercall should be probed using the SMCCC 1.1
> +ARCH_FEATURES mechanism before calling it.
> +
> +PV_LOCK_FEATURES
> +    ============= ========    ==========
> +    Function ID:  (uint32)    0xC5000040
> +    PV_call_id:   (uint32)    The function to query for support.
> +    Return value: (int64)     NOT_SUPPORTED (-1) or SUCCESS (0) if the relevant
> +                              PV-lock feature is supported by the hypervisor.
> +    ============= ========    ==========
> +
> +PV_LOCK_PREEMPTED
> +    ============= ========    ==========
> +    Function ID:  (uint32)    0xC5000041
> +    Return value: (int64)     NOT_SUPPORTED (-1) or SUCCESS (0) if the IPA of
> +                              this vcpu's pv data structure is configured by
> +                              the hypervisor.
> +    ============= ========    ==========

>From the code it looks like there's another argument for this SMC - the
physical address (or IPA) of a struct pvlock_vcpu_state. This structure
also needs to be described as it is part of the ABI.

Steve

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