linux-kernel - Re: [RFC 09/33] KVM: x86: hyper-v: Introduce per-VTL vcpu helpers

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Message-ID: <2a8fddbb-d1d9-4639-a79d-0d32c06d309e@amazon.com>
Date:   Wed, 8 Nov 2023 13:21:47 +0100
From:   Alexander Graf <graf@...zon.com>
To:     Nicolas Saenz Julienne <nsaenz@...zon.com>, <kvm@...r.kernel.org>
CC:     <linux-kernel@...r.kernel.org>, <linux-hyperv@...r.kernel.org>,
        <pbonzini@...hat.com>, <seanjc@...gle.com>, <vkuznets@...hat.com>,
        <anelkz@...zon.com>, <dwmw@...zon.co.uk>, <jgowans@...zon.com>,
        <corbert@....net>, <kys@...rosoft.com>, <haiyangz@...rosoft.com>,
        <decui@...rosoft.com>, <x86@...nel.org>,
        <linux-doc@...r.kernel.org>
Subject: Re: [RFC 09/33] KVM: x86: hyper-v: Introduce per-VTL vcpu helpers


On 08.11.23 12:17, Nicolas Saenz Julienne wrote:
> Introduce two helper functions. The first one queries a vCPU's VTL
> level, the second one, given a struct kvm_vcpu and VTL pair, returns the
> corresponding 'sibling' struct kvm_vcpu at the right VTL.
>
> We keep track of each VTL's state by having a distinct struct kvm_vpcu
> for each level. VTL-vCPUs that belong to the same guest CPU share the
> same physical APIC id, but belong to different APIC groups where the
> apic group represents the vCPU's VTL.
>
> Signed-off-by: Nicolas Saenz Julienne <nsaenz@...zon.com>
> ---
>   arch/x86/kvm/hyperv.h | 18 ++++++++++++++++++
>   1 file changed, 18 insertions(+)
>
> diff --git a/arch/x86/kvm/hyperv.h b/arch/x86/kvm/hyperv.h
> index 2bfed69ba0db..5433107e7cc8 100644
> --- a/arch/x86/kvm/hyperv.h
> +++ b/arch/x86/kvm/hyperv.h
> @@ -23,6 +23,7 @@
>   
>   #include <linux/kvm_host.h>
>   #include "x86.h"
> +#include "lapic.h"
>   
>   /* "Hv#1" signature */
>   #define HYPERV_CPUID_SIGNATURE_EAX 0x31237648
> @@ -83,6 +84,23 @@ static inline struct kvm_hv_syndbg *to_hv_syndbg(struct kvm_vcpu *vcpu)
>   	return &vcpu->kvm->arch.hyperv.hv_syndbg;
>   }
>   
> +static inline struct kvm_vcpu *kvm_hv_get_vtl_vcpu(struct kvm_vcpu *vcpu, int vtl)
> +{
> +	struct kvm *kvm = vcpu->kvm;
> +	u32 target_id = kvm_apic_id(vcpu);
> +
> +	kvm_apic_id_set_group(kvm, vtl, &target_id);
> +	if (vcpu->vcpu_id == target_id)
> +		return vcpu;
> +
> +	return kvm_get_vcpu_by_id(kvm, target_id);
> +}
> +
> +static inline u8 kvm_hv_get_active_vtl(struct kvm_vcpu *vcpu)
> +{
> +	return kvm_apic_group(vcpu);


Shouldn't this check whether VTL is active? If someone wants to use APIC 
groups for a different purpose in the future, they'd suddenly find 
themselves in VTL code paths in other code (such as memory protections), no?

Alex





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