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Message-ID: <0b4c451c6967ffb7bd20b6e5bf7812628aa173c5.camel@infradead.org>
Subject: Re: [PATCH v2] KVM: x86/xen: Update Xen CPUID Leaf 4 (tsc info)
 sub-leaves, if present
From:   David Woodhouse <dwmw2@infradead.org>
To:     Paul Durrant <pdurrant@amazon.com>, x86@kernel.org,
        kvm@vger.kernel.org, linux-kernel@vger.kernel.org
Cc:     Paolo Bonzini <pbonzini@redhat.com>,
        Sean Christopherson <seanjc@google.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>
Date:   Wed, 22 Jun 2022 14:47:56 +0100
In-Reply-To: <20220622095750.30563-1-pdurrant@amazon.com>
References: <20220622095750.30563-1-pdurrant@amazon.com>
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On Wed, 2022-06-22 at 10:57 +0100, Paul Durrant wrote:
> +void kvm_xen_set_cpuid(struct kvm_vcpu *vcpu)
> +{
> +	u32 base =3D 0;
> +	u32 function;
> +
> +	vcpu->arch.xen.tsc_info_1 =3D NULL;
> +	vcpu->arch.xen.tsc_info_2 =3D NULL;
> +
> +	for_each_possible_hypervisor_cpuid_base(function) {
> +		struct kvm_cpuid_entry2 *entry =3D kvm_find_cpuid_entry(vcpu, function=
, 0);
> +
> +		if (entry &&
> +		    entry->ebx =3D=3D XEN_CPUID_SIGNATURE_EBX &&
> +		    entry->ecx =3D=3D XEN_CPUID_SIGNATURE_ECX &&
> +		    entry->edx =3D=3D XEN_CPUID_SIGNATURE_EDX) {
> +			base =3D function;
> +			break;
> +		}
> +	}

Please make it return if entry->eax < 3 here. There probably aren't any
*good* reasons why a leaf at 0x40000x03 would exist and belong to some
other entity (Hyper-V, etc.). Those other extra ranges of CPUID are
generally aligned at multiples of 0x100, not just the *next* available
leaf.

But I don't care. Unless eax on the main Xen leaf is 3 or more, the
leaf at N+3 isn't yours to reason about :)

> +	if (!base)
> +		return;
> +
> +	function =3D base | XEN_CPUID_LEAF(3);
> +	vcpu->arch.xen.tsc_info_1 =3D kvm_find_cpuid_entry(vcpu, function, 1);
> +	vcpu->arch.xen.tsc_info_2 =3D kvm_find_cpuid_entry(vcpu, function, 2);
> +}
> +

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