linux-kernel - Re: [PATCH v2] KVM: x86/xen: Update Xen CPUID Leaf 4 (tsc info) sub-leaves, if present

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Message-ID: <0b4c451c6967ffb7bd20b6e5bf7812628aa173c5.camel@infradead.org>
Date:   Wed, 22 Jun 2022 14:47:56 +0100
From:   David Woodhouse <dwmw2@...radead.org>
To:     Paul Durrant <pdurrant@...zon.com>, x86@...nel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v2] KVM: x86/xen: Update Xen CPUID Leaf 4 (tsc info)
 sub-leaves, if present

On Wed, 2022-06-22 at 10:57 +0100, Paul Durrant wrote:
> +void kvm_xen_set_cpuid(struct kvm_vcpu *vcpu)
> +{
> +	u32 base = 0;
> +	u32 function;
> +
> +	vcpu->arch.xen.tsc_info_1 = NULL;
> +	vcpu->arch.xen.tsc_info_2 = NULL;
> +
> +	for_each_possible_hypervisor_cpuid_base(function) {
> +		struct kvm_cpuid_entry2 *entry = kvm_find_cpuid_entry(vcpu, function, 0);
> +
> +		if (entry &&
> +		    entry->ebx == XEN_CPUID_SIGNATURE_EBX &&
> +		    entry->ecx == XEN_CPUID_SIGNATURE_ECX &&
> +		    entry->edx == XEN_CPUID_SIGNATURE_EDX) {
> +			base = 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 = base | XEN_CPUID_LEAF(3);
> +	vcpu->arch.xen.tsc_info_1 = kvm_find_cpuid_entry(vcpu, function, 1);
> +	vcpu->arch.xen.tsc_info_2 = kvm_find_cpuid_entry(vcpu, function, 2);
> +}
> +

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