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From:   Raghavendra Rao Ananta <rananta@google.com>
Date:   Mon, 10 Jan 2022 15:40:35 -0800
Message-ID: <CAJHc60yY9qH5_r09Tz2fhWr+tT+i7RnKhchBuEePCKnos52kwA@mail.gmail.com>
Subject: Re: [RFC PATCH v3 03/11] KVM: Introduce KVM_CAP_ARM_HVC_FW_REG_BMAP
To:     Reiji Watanabe <reijiw@google.com>
Cc:     Marc Zyngier <maz@kernel.org>, Andrew Jones <drjones@redhat.com>,
        James Morse <james.morse@arm.com>,
        Alexandru Elisei <alexandru.elisei@arm.com>,
        Suzuki K Poulose <suzuki.poulose@arm.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>, Peter Shier <pshier@google.com>,
        Ricardo Koller <ricarkol@google.com>,
        Oliver Upton <oupton@google.com>,
        Jing Zhang <jingzhangos@google.com>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        kvmarm@lists.cs.columbia.edu, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org
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On Fri, Jan 7, 2022 at 9:40 PM Reiji Watanabe <reijiw@google.com> wrote:
>
> Hi Raghu,
>
> On Tue, Jan 4, 2022 at 11:49 AM Raghavendra Rao Ananta
> <rananta@google.com> wrote:
> >
> > Introduce the KVM ARM64 capability, KVM_CAP_ARM_HVC_FW_REG_BMAP,
> > to indicate the support for psuedo-firmware bitmap extension.
> > Each of these registers holds a feature-set exposed to the guest
> > in the form of a bitmap. If supported, a simple 'read' of the
> > capability should return the number of psuedo-firmware registers
> > supported. User-space can utilize this to discover the registers.
> > It can further explore or modify the features using the classical
> > GET/SET_ONE_REG interface.
> >
> > Signed-off-by: Raghavendra Rao Ananta <rananta@google.com>
> > ---
> >  Documentation/virt/kvm/api.rst | 21 +++++++++++++++++++++
> >  include/uapi/linux/kvm.h       |  1 +
> >  2 files changed, 22 insertions(+)
> >
> > diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> > index aeeb071c7688..646176537f2c 100644
> > --- a/Documentation/virt/kvm/api.rst
> > +++ b/Documentation/virt/kvm/api.rst
> > @@ -6925,6 +6925,27 @@ indicated by the fd to the VM this is called on.
> >  This is intended to support intra-host migration of VMs between userspace VMMs,
> >  upgrading the VMM process without interrupting the guest.
> >
> > +7.30 KVM_CAP_ARM_HVC_FW_REG_BMAP
>
> IMHO, instead of including its format of the register in the name,
> including its purpose/function in the name might be better.
> e.g. KVM_CAP_ARM_HVC_FEATURE_REG ?
> (Feature fields don't necessarily have to be in a bitmap format
>  if they don't fit well although I'm not sure if we have such fields.)
>
Well we do have registers, KVM_REG_ARM_PSCI_VERSION for instance,
that's not covered by this CAP. But sure, I can explicitly add
'FEATURES' to the name. I also wanted to explicitly convey that we are
covering the *bitmapped* firmware registers here. But not sure if
appending 'BMAP' might give an impression that the CAP itself is
bitmapped.
Do you think KVM_CAP_ARM_HVC_BMAP_FEAT_REG is better?

> > +
> > +:Architectures: arm64
> > +:Parameters: None
> > +:Returns: Number of psuedo-firmware registers supported
>
> Looking at patch-4, the return value of this would be the number of
> pseudo-firmware *bitmap* registers supported.
> BTW, "4.68 KVM_SET_ONE_REG" in the doc uses the word "arm64 firmware
> pseudo-registers".  It would be nicer to use the same term.
>
Nice catch. I'll fix it here in apr.rst.
> > +
> > +This capability indicates that KVM for arm64 supports the psuedo-firmware
> > +register bitmap extension. Each of these registers represent the features
> > +supported by a particular type in the form of a bitmap. By default, these
> > +registers are set with the upper limit of the features that are supported.
> > +
> > +The registers can be accessed via the standard SET_ONE_REG and KVM_GET_ONE_REG
> > +interfaces. The user-space is expected to read the number of these registers
> > +available by reading KVM_CAP_ARM_HVC_FW_REG_BMAP, read the current bitmap
> > +configuration via GET_ONE_REG for each register, and then write back the
> > +desired bitmap of features that it wishes the guest to see via SET_ONE_REG.
> > +
> > +Note that KVM doesn't allow the user-space to modify these registers after
> > +the VM (any of the vCPUs) has started running.
>
> Since even if KVM_RUN fails, and the VM hasn't started yet,
> it will get immutable. So, "after any of the vCPUs run KVM_RUN."
> might be more clear ?
>
Sure, that's probably more clear. I'll fix it.

Regards,
Raghavendra

> Thanks,
> Reiji
>
>
>
> > +
> >  8. Other capabilities.
> >  ======================
> >
> > diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> > index 1daa45268de2..209b43dbbc3c 100644
> > --- a/include/uapi/linux/kvm.h
> > +++ b/include/uapi/linux/kvm.h
> > @@ -1131,6 +1131,7 @@ struct kvm_ppc_resize_hpt {
> >  #define KVM_CAP_EXIT_ON_EMULATION_FAILURE 204
> >  #define KVM_CAP_ARM_MTE 205
> >  #define KVM_CAP_VM_MOVE_ENC_CONTEXT_FROM 206
> > +#define KVM_CAP_ARM_HVC_FW_REG_BMAP 207
> >
> >  #ifdef KVM_CAP_IRQ_ROUTING
> >
> > --
> > 2.34.1.448.ga2b2bfdf31-goog
> >