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Message-ID: <b4ad6c2b67dcb9f17cf14f247dae9bab880b346c.camel@infradead.org>
Subject: Re: [PATCH RFC 01/39] KVM: x86: fix Xen hypercall page msr handling
From:   David Woodhouse <dwmw2@infradead.org>
To:     Paolo Bonzini <pbonzini@redhat.com>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        Joao Martins <joao.m.martins@oracle.com>
Cc:     kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
        Ankur Arora <ankur.a.arora@oracle.com>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Radim =?UTF-8?Q?Kr=C4=8Dm=C3=A1=C5=99?= <rkrcmar@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>, x86@kernel.org
Date:   Mon, 30 Nov 2020 11:27:43 +0000
In-Reply-To: <51b7771a-0b76-37e3-a80b-e372a7467aca@redhat.com>
References: <20190220201609.28290-1-joao.m.martins@oracle.com>
         <20190220201609.28290-2-joao.m.martins@oracle.com>
         <20190222013008.GG7224@linux.intel.com>
         <44b102eb-ea74-7f19-3f4a-41dfc298d372@redhat.com>
         <188a300f8314dd30a3a71857f63f144a3ce69950.camel@infradead.org>
         <51b7771a-0b76-37e3-a80b-e372a7467aca@redhat.com>
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On Mon, 2020-11-30 at 12:03 +0100, Paolo Bonzini wrote:
> You can use CPUID too (search for Hv#1 in leaf 0x40000000)?

That's leaf 0x40000001. Which is also the leaf used for Xen to indicate
the Xen version. So as long as we don't pretend to be Xen version
12759.30280 I suppose that's OK.

Or we could just check leaf 0x40000000 for 'Microsoft Hv'? Or both.

How about...

#define HYPERV_CPUID_INTERFACE_MAGIC 0x31237648 /* 'Hv#1' */

static inline bool guest_cpu_has_hyperv(struct kvm_vcpu *vcpu)
{
    	struct kvm_hv *hv =3D &vcpu->kvm->arch.hyperv;
	const struct kvm_cpuid_entry2 *entry;

        /*
         * The call to kvm_find_cpuid_entry() is a bit much to put on
 =20
* the fast path of some of the Hyper-V MSRs, so bypass it if
         * the guest OS ID has already been set.
         */
	if (hv->hv_guest_os_id)
                return true;

        entry =3D kvm_find_cpuid_entry(vcpu, HYPERV_CPUID_INTERFACE, 0x0);
        return entry && entry->eax =3D=3D HYPERV_CPUID_INTERFACE_MAGIC;
}

I wonder if this is overengineering when 'let Xen take precedence'
works well enough?

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