Re: [PATCH v2 0/1] x86/kvm/hyper-v: Add support to SYNIC exit on EOM

From: Jon Doron <arilou@gmail.com>
To: Roman Kagan <rvkagan@yandex-team.ru>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	kvm@vger.kernel.org, linux-hyperv@vger.kernel.org
Subject: Re: [PATCH v2 0/1] x86/kvm/hyper-v: Add support to SYNIC exit on EOM
Date: Tue, 5 May 2020 13:38:21 +0300	[thread overview]
Message-ID: <20200505103821.GB2862@jondnuc> (raw)
In-Reply-To: <20200505080158.GA400685@rvkaganb>

On 05/05/2020, Roman Kagan wrote:
>On Mon, May 04, 2020 at 05:55:10PM +0200, Vitaly Kuznetsov wrote:
>> Roman Kagan <rvkagan@yandex-team.ru> writes:
>>
>> > On Sat, Apr 25, 2020 at 09:16:37AM +0300, Jon Doron wrote:
>> >
>> >> If that's indeed the case then probably the only thing needs fixing in my
>> >> scenario is in QEMU where it should not really care for the SCONTROL if it's
>> >> enabled or not.
>> >
>> > Right.  However, even this shouldn't be necessary as SeaBIOS from that
>> > branch would enable SCONTROL and leave it that way when passing the
>> > control over to the bootloader, so, unless something explicitly clears
>> > SCONTROL, it should remain set thereafter.  I'd rather try going ahead
>> > with that scheme first, because making QEMU ignore SCONTROL appears to
>> > violate the spec.
>>
>> FWIW, I just checked 'genuine' Hyper-V 2016 with
>>
>> diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
>> index fd51bac11b46..c5ea759728d9 100644
>> --- a/arch/x86/hyperv/hv_init.c
>> +++ b/arch/x86/hyperv/hv_init.c
>> @@ -314,10 +314,14 @@ void __init hyperv_init(void)
>>         u64 guest_id, required_msrs;
>>         union hv_x64_msr_hypercall_contents hypercall_msr;
>>         int cpuhp, i;
>> +       u64 val;
>>
>>         if (x86_hyper_type != X86_HYPER_MS_HYPERV)
>>                 return;
>>
>> +       hv_get_synic_state(val);
>> +       printk("Hyper-V: SCONTROL state: %llx\n", val);
>> +
>>         /* Absolutely required MSRs */
>>         required_msrs = HV_X64_MSR_HYPERCALL_AVAILABLE |
>>                 HV_X64_MSR_VP_INDEX_AVAILABLE;
>
>Thanks for having done this check!
>
>> and it seems the default state of HV_X64_MSR_SCONTROL is '1', we should
>> probably do the same.
>
>This is the state the OS sees, after the firmware.  You'd see the same
>with QEMU/KVM if you used Hyper-V-aware SeaBIOS or OVMF.
>
>> Is there any reason to *not* do this in KVM when
>> KVM_CAP_HYPERV_SYNIC[,2] is enabled?
>
>Yes there is: quoting Hyper-V TLFS v6.0 11.8.1:
>
>  At virtual processor creation time and upon processor reset, the value
>  of this SCONTROL (SynIC control register) is 0x0000000000000000. Thus,
>  message queuing and event flag notifications will be disabled.
>
>And, even if we decide to violate the spec it's better done in
>userspace, loading the initial value and adjusting the synic state at
>vcpu reset.
>
>However leaving it up to the guest (firmware or OS) looks more natural
>to me.
>
>Thanks,
>Roman.

I under where you are coming from in the idea of leaving it to the OS 
but I think in this specific case it does not make much sense, after all 
HyperV has it's own proprietary BIOS which Windows assumes has setup 
some of the MSRs, since we dont have that BIOS we need to "emulate" it's 
behaviour.

I also feel like the best approach should be in QEMU in case VMBus 
device exists it will also setup the SCONTROL to ENABLED, this way you 
are not bound to have a special BIOS in case you have decided to use 
HyperV advanced features like VMBus.

Cheers,
-- Jon.