linux-kernel - Re: [v2 1/6] KVM: arm64: Document PV-lock interface

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Date:   Mon, 07 Nov 2022 17:56:49 +0000
From:   Punit Agrawal <punit.agrawal@...edance.com>
To:     Usama Arif <usama.arif@...edance.com>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        kvmarm@...ts.cs.columbia.edu, kvm@...r.kernel.org,
        linux-doc@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, linux@...linux.org.uk,
        yezengruan@...wei.com, catalin.marinas@....com, will@...nel.org,
        maz@...nel.org, steven.price@....com, mark.rutland@....com,
        bagasdotme@...il.com, fam.zheng@...edance.com,
        liangma@...ngbit.com, punit.agrawal@...edance.com
Subject: Re: [v2 1/6] KVM: arm64: Document PV-lock interface

Hi Usama,

Usama Arif <usama.arif@...edance.com> writes:

> Introduce a paravirtualization interface for KVM/arm64 to obtain whether
> the VCPU is currently running or not.
>
> The PV lock structure of the guest is allocated by user space.
>
> A hypercall interface is provided for the guest to interrogate the
> location of the shared memory structures.
>
> Signed-off-by: Zengruan Ye <yezengruan@...wei.com>
> Signed-off-by: Usama Arif <usama.arif@...edance.com>
> ---
>  Documentation/virt/kvm/arm/index.rst    |  1 +
>  Documentation/virt/kvm/arm/pvlock.rst   | 52 +++++++++++++++++++++++++
>  Documentation/virt/kvm/devices/vcpu.rst | 25 ++++++++++++
>  3 files changed, 78 insertions(+)
>  create mode 100644 Documentation/virt/kvm/arm/pvlock.rst
>
> diff --git a/Documentation/virt/kvm/arm/index.rst b/Documentation/virt/kvm/arm/index.rst
> index e84848432158..b8499dc00a6a 100644
> --- a/Documentation/virt/kvm/arm/index.rst
> +++ b/Documentation/virt/kvm/arm/index.rst
> @@ -10,4 +10,5 @@ ARM
>     hyp-abi
>     hypercalls
>     pvtime
> +   pvlock
>     ptp_kvm
> diff --git a/Documentation/virt/kvm/arm/pvlock.rst b/Documentation/virt/kvm/arm/pvlock.rst
> new file mode 100644
> index 000000000000..d3c391b16d36
> --- /dev/null
> +++ b/Documentation/virt/kvm/arm/pvlock.rst
> @@ -0,0 +1,52 @@
> +.. SPDX-License-Identifier: GPL-2.0
> +
> +Paravirtualized lock support for arm64
> +======================================
> +
> +KVM/arm64 provides a hypervisor service call for paravirtualized guests to
> +determine whether a VCPU is currently running or not.
> +
> +A new SMCCC compatible hypercall is defined:
> +
> +* ARM_SMCCC_VENDOR_HYP_KVM_PV_LOCK_FUNC_ID:   0xC6000002
> +
> +ARM_SMCCC_VENDOR_HYP_KVM_PV_LOCK_FUNC_ID
> +
> +    ============= ========    ==========================================
> +    Function ID:  (uint32)    0xC6000002
> +    Return value: (int64)     IPA of the pv lock data structure for this
> +                              VCPU. On failure:
> +                              NOT_SUPPORTED (-1)
> +    ============= ========    ==========================================
> +
> +The IPA returned by PV_LOCK_PREEMPTED should be mapped by the guest as normal
> +memory with inner and outer write back caching attributes, in the inner
> +shareable domain.
> +
> +PV_LOCK_PREEMPTED returns the structure for the calling VCPU.
> +
> +PV lock state
> +-------------
> +
> +The structure pointed to by the PV_LOCK_PREEMPTED hypercall is as follows:
> +
> ++-----------+-------------+-------------+---------------------------------+
> +| Field     | Byte Length | Byte Offset | Description                     |
> ++===========+=============+=============+=================================+
> +| preempted |      8      |      0      | Indicate if the VCPU that owns  |
> +|           |             |             | this struct is running or not.  |
> +|           |             |             | Non-zero values mean the VCPU   |
> +|           |             |             | has been preempted. Zero means  |
> +|           |             |             | the VCPU is not preempted.      |
> ++-----------+-------------+-------------+---------------------------------+
> +
> +The preempted field will be updated to 1 by the hypervisor prior to scheduling
> +a VCPU. When the VCPU is scheduled out, the preempted field will be updated
> +to 0 by the hypervisor.

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