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Subject: Re: [PATCH RFC 01/39] KVM: x86: fix Xen hypercall page msr handling
To:     David Woodhouse <dwmw2@infradead.org>,
        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>,
        =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <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
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>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <51b7771a-0b76-37e3-a80b-e372a7467aca@redhat.com>
Date:   Mon, 30 Nov 2020 12:03:30 +0100
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On 30/11/20 11:39, David Woodhouse wrote:
> ... except that's a bit icky because that trick of falling through to
> the default case only works for *one* case statement. And more to the
> point, the closest thing I can find to a 'kvm_hyperv_enabled()' flag is
> what we do for setting the HV_X64_MSR_HYPERCALL_ENABLE flag... which is
> based on whether the hv_guest_os_id is set, which in turn is done by
> writing one of these MSRs 

You can use CPUID too (search for Hv#1 in leaf 0x40000000)?

Paolo

> I suppose we could disable them just by letting Xen take precedence, if
> kvm->arch.xen_hvm_config.msr == HV_X64_MSR_GUEST_OS_ID. But that's
> basically what Joao's patch already does. It doesn't disable the
> *other* Hyper-V MSRs except for the one Xen 'conflicts' with, but I
> don't think that matters.