[RFC PATCH 0/3] Target CPU=Host implementation for KVM ARM/ARM64

[Anup Patel]

On Thu, Sep 5, 2013 at 8:21 PM, Peter Maydell <peter.maydell at linaro.org> wrote:
> On 5 September 2013 15:45, Anup Patel <anup.patel at linaro.org> wrote:
>> It will be very useful for user space if it has a method for specifying
>> KVM ARM/ARM64 to give a VCPU with target type suitable to underlying host
>> but with particular set of features.
>>
>> In other words, user space might not be interested in having a particular
>> target type for VCPU but it will certainely want particular set of features
>> in the VCPU.
>>
>> The patch tries to implement above described method of specifying VCPU
>> target CPU=Host from user space by extending the KVM_ARM_VCPU_INIT ioctl
>> and having a dummy target KVM_ARM_TARGET_HOST which means Target CPU
>> same as (or suitable to) underlying host.
>
> I thought the consensus on the call last week was
> to have an ioctl for "get best CPU for this host"
> and then for userspace to pass that value to
> VCPU_INIT ?

I had considered having a separate ioctl for "get best CPU for this host"
(as per KVM call minutes) but the same thing is possible with existing
KVM_ARM_VCPU_INIT so why not extend this ioctl. Also, I was finding
it hard to come up with a use case where having a separate ioctl for
"get best CPU for this host" would be better for user space.

Further, the approach proposed by this patch also makes the
KVM_ARM_VCPU_INIT ioctl inline with what we do for KVM x86 that is
provide only a set of features user space wish and KVM x86 will try to
provide most of those features with a virtual CPU suitable to host.

Another advantage I saw in extending KVM_ARM_VCPU_INIT ioctl is
backward compatibility with current semantics. In other words, this patch
does not break current KVMTOOL/QEMU and they can implement
"-cpu host" whenever they wish without using any additional ioctl.

PFA, KVMTOOL patch which I used to test KVM_ARM_TARGET_HOST.

>
> -- PMM
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--Anup
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