linux-kernel - Re: [PATCH 5/5] x86/kvm: Add KVM_FEATURE_MSI_EXT_DEST

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Message-ID: <87blhcx6qz.fsf@nanos.tec.linutronix.de>
Date:   Thu, 08 Oct 2020 14:05:40 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     David Woodhouse <dwmw2@...radead.org>, x86@...nel.org
Cc:     kvm <kvm@...r.kernel.org>, Paolo Bonzini <pbonzini@...hat.com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 5/5] x86/kvm: Add KVM_FEATURE_MSI_EXT_DEST_ID

On Wed, Oct 07 2020 at 13:20, David Woodhouse wrote:
> From: David Woodhouse <dwmw@...zon.co.uk>
>
> This allows the host to indicate that IOAPIC and MSI emulation supports
> 15-bit destination IDs, allowing up to 32768 CPUs without interrupt
> remapping.
>
> cf. https://patchwork.kernel.org/patch/11816693/ for qemu
>
> Signed-off-by: David Woodhouse <dwmw@...zon.co.uk>
> Acked-by: Paolo Bonzini <pbonzini@...hat.com>
> ---
>  Documentation/virt/kvm/cpuid.rst     | 4 ++++
>  arch/x86/include/uapi/asm/kvm_para.h | 1 +
>  arch/x86/kernel/kvm.c                | 6 ++++++
>  3 files changed, 11 insertions(+)
>
> diff --git a/Documentation/virt/kvm/cpuid.rst b/Documentation/virt/kvm/cpuid.rst
> index a7dff9186bed..1726b5925d2b 100644
> --- a/Documentation/virt/kvm/cpuid.rst
> +++ b/Documentation/virt/kvm/cpuid.rst
> @@ -92,6 +92,10 @@ KVM_FEATURE_ASYNC_PF_INT          14          guest checks this feature bit
>                                                async pf acknowledgment msr
>                                                0x4b564d07.
>  
> +KVM_FEATURE_MSI_EXT_DEST_ID       15          guest checks this feature bit
> +                                              before using extended destination
> +                                              ID bits in MSI address
> bits 11-5.

Why MSI_EXT_DEST_ID? It's enabling that for MSI and IO/APIC. The
underlying mechanism might be the same, but APIC_EXT_DEST_ID is more
general and then you might also make the explanation of that bit match
the changelog.

Thanks,

        tglx