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Subject: Re: [PATCH v2 3/4] x86/kvm/hyper-v: direct mode for synthetic timers
To:     Roman Kagan <rkagan@virtuozzo.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>, kvm@vger.kernel.org,
        =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        linux-kernel@vger.kernel.org,
        "K. Y. Srinivasan" <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>, x86@kernel.org,
        "Michael Kelley (EOSG)" <Michael.H.Kelley@microsoft.com>
References: <20181126154732.23025-1-vkuznets@redhat.com>
 <20181126154732.23025-4-vkuznets@redhat.com>
 <e565638e-d6cc-bbd9-f99e-c835ef1be5b7@redhat.com>
 <20181127083706.GB16047@rkaganb.sw.ru>
From:   Paolo Bonzini <pbonzini@redhat.com>
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Message-ID: <9d0e9796-b71e-9437-8cd4-a898fd503871@redhat.com>
Date:   Tue, 27 Nov 2018 14:54:21 +0100
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On 27/11/18 09:37, Roman Kagan wrote:
> On Mon, Nov 26, 2018 at 05:44:24PM +0100, Paolo Bonzini wrote:
>> On 26/11/18 16:47, Vitaly Kuznetsov wrote:
>>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>>> index 5cd5647120f2..b21b5ceb8d26 100644
>>> --- a/arch/x86/kvm/x86.c
>>> +++ b/arch/x86/kvm/x86.c
>>> @@ -2997,6 +2997,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
>>>  	case KVM_CAP_HYPERV_TLBFLUSH:
>>>  	case KVM_CAP_HYPERV_SEND_IPI:
>>>  	case KVM_CAP_HYPERV_ENLIGHTENED_VMCS:
>>> +	case KVM_CAP_HYPERV_STIMER_DIRECT:
>>>  	case KVM_CAP_PCI_SEGMENT:
>>>  	case KVM_CAP_DEBUGREGS:
>>>  	case KVM_CAP_X86_ROBUST_SINGLESTEP:
>>> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
>>> index 2b7a652c9fa4..b8da14cee8e5 100644
>>> --- a/include/uapi/linux/kvm.h
>>> +++ b/include/uapi/linux/kvm.h
>>> @@ -975,6 +975,7 @@ struct kvm_ppc_resize_hpt {
>>>  #define KVM_CAP_HYPERV_ENLIGHTENED_VMCS 163
>>>  #define KVM_CAP_EXCEPTION_PAYLOAD 164
>>>  #define KVM_CAP_ARM_VM_IPA_SIZE 165
>>> +#define KVM_CAP_HYPERV_STIMER_DIRECT 166
>>
>> I wonder if all these capabilities shouldn't be replaced by a single
>> KVM_GET_HYPERV_SUPPORTED_CPUID ioctl, or something like that.
> 
> Hmm, why?  Are we running short of cap numbers?
> 
> Capabilities are a well-established and unambiguous negotiation
> mechanism, why invent another one?  Besides, not all features map
> conveniently onto cpuid bits, e.g. currently we have two versions of
> SynIC support, which differ in the way the userspace deals with it, but
> not in the cpuid bits we expose in the guest.  IMO such an ioctl would
> bring more complexity rather than less.

Yes, in that case (for bugfixes basically) you'd have to use
capabilities.  But if the feature is completely hidden to userspace
except for the CPUID bit, it seems like a ioctl would be more consistent
with the rest of the KVM API.

Paolo