linux-kernel - Re: [PATCH v2 3/3] x86/kvm/hyper-v: inject #GP only when invalid SINTx vector is unmasked

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Date:   Thu, 1 Mar 2018 17:59:35 +0300
From:   Roman Kagan <rkagan@...tuozzo.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org, x86@...nel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        "Michael Kelley (EOSG)" <Michael.H.Kelley@...rosoft.com>,
        "Denis V . Lunev" <den@...nvz.org>
Subject: Re: [PATCH v2 3/3] x86/kvm/hyper-v: inject #GP only when invalid
 SINTx vector is unmasked

On Thu, Mar 01, 2018 at 03:15:14PM +0100, Vitaly Kuznetsov wrote:
> Hyper-V 2016 on KVM with SynIC enabled doesn't boot with the following
> trace:
> 
>     kvm_entry:            vcpu 0
>     kvm_exit:             reason MSR_WRITE rip 0xfffff8000131c1e5 info 0 0
>     kvm_hv_synic_set_msr: vcpu_id 0 msr 0x40000090 data 0x10000 host 0
>     kvm_msr:              msr_write 40000090 = 0x10000 (#GP)
>     kvm_inj_exception:    #GP (0x0)
> 
> KVM acts according to the following statement from TLFS:
> 
> "
> 11.8.4 SINTx Registers
> ...
> Valid values for vector are 16-255 inclusive. Specifying an invalid
> vector number results in #GP.
> "
> 
> However, I checked and genuine Hyper-V doesn't #GP when we write 0x10000
> to SINTx. I checked with Microsoft and they confirmed that if either the
> Masked bit (bit 16) or the Polling bit (bit 18) is set to 1, then they
> ignore the value of Vector. Make KVM act accordingly.
> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> ---
> Changes since v1:
> - Drop 'polling' bit check for now as we don't support this mode. We'll
>   need to bring some form of this check back when polling mode is
>   implemented [Roman Kagan].
> - Add a comment explaining "!host && !masked" in synic_set_sint()
> ---
>  arch/x86/kvm/hyperv.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)

Reviewed-by: Roman Kagan <rkagan@...tuozzo.com>