Re: [PATCH v2 4/7] KVM: x86: hyper-v: always advertise HV_STIMER_DIRECT_MODE_AVAILABLE

From: Vitaly Kuznetsov <vkuznets@redhat.com>
To: Paolo Bonzini <pbonzini@redhat.com>, kvm@vger.kernel.org
Cc: Sean Christopherson <sean.j.christopherson@intel.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Jim Mattson <jmattson@google.com>,
	Eduardo Habkost <ehabkost@redhat.com>,
	Jon Doron <arilou@gmail.com>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 4/7] KVM: x86: hyper-v: always advertise HV_STIMER_DIRECT_MODE_AVAILABLE
Date: Tue, 29 Sep 2020 13:26:36 +0200	[thread overview]
Message-ID: <87lfgsbz83.fsf@vitty.brq.redhat.com> (raw)
In-Reply-To: <34ea6c7c-6c10-5cdb-de75-6b4afc023dd2@redhat.com>

Paolo Bonzini <pbonzini@redhat.com> writes:

> On 29/09/20 12:36, Vitaly Kuznetsov wrote:
>>> Sorry for the late reply.  I think this is making things worse.  It's
>>> obviously okay to add a system KVM_GET_SUPPORTED_HV_CPUID, and I guess
>>> it makes sense to have bits in there that require to enable a
>>> capability.  For example, KVM_GET_SUPPORTED_CPUID has a couple bits such
>>> as X2APIC, that we return even if they require in-kernel irqchip.
>>>
>>> For the vCPU version however we should be able to copy the returned
>>> leaves to KVM_SET_CPUID2, meaning that unsupported features should be
>>> masked.
>> What I don't quite like about exposing HV_STIMER_DIRECT_MODE_AVAILABLE
>> conditionally is that we're requiring userspace to have a certain
>> control flow: first, it needs to create irqchip and only then call
>> KVM_GET_SUPPORTED_HV_CPUID or it won't know that
>> HV_STIMER_DIRECT_MODE_AVAILABLE is supported. 
>> 
>> Also, are you only concerned about HV_STIMER_DIRECT_MODE_AVAILABLE? E.g.
>> PATCH3 of this series is somewhat similar, it exposes eVMCS even when
>> the corresponding CAP wasn't enabled.
>
> All of them, but this was only about the vCPU ioctl.  I agree with you
> that the system ioctl should return everything unconditionally.
>
> But perhaps the best thing to do is to deprecate the vCPU ioctl and just
> leave it as is with all its quirks.
>

Ok, I'll do exactly that. I'm not sure if there are any
KVM_GET_SUPPORTED_HV_CPUID users out there bisedes QEMU/selftest but
let's take the 'safest' approach.

-- 
Vitaly