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

[Roman Kagan <rvkagan@yandex-team.ru>]

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.