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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 Krčmář" <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
Subject: Re: [PATCH RFC 01/39] KVM: x86: fix Xen hypercall page msr handling
Date: Mon, 30 Nov 2020 10:39:10 +0000	[thread overview]
Message-ID: <188a300f8314dd30a3a71857f63f144a3ce69950.camel@infradead.org> (raw)
In-Reply-To: <44b102eb-ea74-7f19-3f4a-41dfc298d372@redhat.com>

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On Fri, 2019-02-22 at 13:51 +0100, Paolo Bonzini wrote:
> On 22/02/19 02:30, Sean Christopherson wrote:
> >        if (kvm_advertise_kvm()) {
> >                if (<handle kvm msr>)
> >                        return ...;
> >        } else if (kvm_advertise_hyperv()) {
> >                if (<handle hyperv msr>)
> >                        return ...;
> >        } else if (kvm_advertise_xen()) {
> >                if (<handle xen msrs>)
> >                        return ...;
> >        }
> > 
> >        <fall through to main switch statement>
> > 
> > Obviously assumes KVM only advertises itself as one hypervisor, and so
> > the ordering is arbitrary.
> 
> No, KVM can advertise as both KVM and Hyper-V.  CPUID 0x40000000 is used
> for Hyper-V, while 0x40000100 is used for KVM.  The MSRs do not conflict.

The MSRs *do* conflict. Kind of...

Xen uses MSR 0x40000000 (not to be conflated with CPUID leaf
0x40000000) for the "write hypercall page" request.

That conflicts with Hyper-V's HV_X64_MSR_GUEST_OS_ID.

So when the Hyper-V extensions are enabled, Xen moves its own MSR to
0x40000200 to avoid the conflict.

The problem is that KVM services the Hyper-V MSRs unconditionally in
the switch statement in kvm_set_msr_common(). So if the Xen MSR is set
to 0x40000000 and Hyper-V is *not* enabled, the Hyper-V support still
stops the Xen MSR from working.

Joao's patch fixes that.

A nicer alternative might be to disable the Hyper-V MSRs when they
shouldn't be there. Something like...

--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2788,15 +2788,6 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
                 * the need to ignore the workaround.
                 */
                break;
-       case HV_X64_MSR_GUEST_OS_ID ... HV_X64_MSR_SINT15:
-       case HV_X64_MSR_CRASH_P0 ... HV_X64_MSR_CRASH_P4:
-       case HV_X64_MSR_CRASH_CTL:
-       case HV_X64_MSR_STIMER0_CONFIG ... HV_X64_MSR_STIMER3_COUNT:
-       case HV_X64_MSR_REENLIGHTENMENT_CONTROL:
-       case HV_X64_MSR_TSC_EMULATION_CONTROL:
-       case HV_X64_MSR_TSC_EMULATION_STATUS:
-               return kvm_hv_set_msr_common(vcpu, msr, data,
-                                            msr_info->host_initiated);
        case MSR_IA32_BBL_CR_CTL3:
                /* Drop writes to this legacy MSR -- see rdmsr
                 * counterpart for further detail.
@@ -2829,6 +2820,18 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
                        return 1;
                vcpu->arch.msr_misc_features_enables = data;
                break;
+       case HV_X64_MSR_GUEST_OS_ID ... HV_X64_MSR_SINT15:
+       case HV_X64_MSR_CRASH_P0 ... HV_X64_MSR_CRASH_P4:
+       case HV_X64_MSR_CRASH_CTL:
+       case HV_X64_MSR_STIMER0_CONFIG ... HV_X64_MSR_STIMER3_COUNT:
+       case HV_X64_MSR_REENLIGHTENMENT_CONTROL:
+       case HV_X64_MSR_TSC_EMULATION_CONTROL:
+       case HV_X64_MSR_TSC_EMULATION_STATUS:
+               if (kvm_hyperv_enabled(vcpu->kvm)) {
+                       return kvm_hv_set_msr_common(vcpu, msr, data,
+                                                    msr_info->host_initiated);
+               }
+               /* fall through */
        default:
                if (msr && (msr == vcpu->kvm->arch.xen_hvm_config.msr))
                        return xen_hvm_config(vcpu, data);


... 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 :)

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.

The patch stands alone to correct the *existing* functionality of
KVM_XEN_HVM_CONFIG, regardless of the additional functionality being
proposed in the rest of the series that followed it.

Reviewed-by: David Woodhouse <dwmw@amazon.co.uk>
Cc: stable@vger.kernel.org



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  reply	other threads:[~2020-11-30 10:40 UTC|newest]

Thread overview: 126+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-02-20 20:15 [PATCH RFC 00/39] x86/KVM: Xen HVM guest support Joao Martins
2019-02-20 20:15 ` [PATCH RFC 01/39] KVM: x86: fix Xen hypercall page msr handling Joao Martins
2019-02-22  1:30   ` Sean Christopherson
2019-02-22 11:47     ` Joao Martins
2019-02-22 12:51     ` Paolo Bonzini
2020-11-30 10:39       ` David Woodhouse [this message]
2020-11-30 11:03         ` Paolo Bonzini
2020-11-30 11:27           ` David Woodhouse
2019-02-20 20:15 ` [PATCH RFC 02/39] KVM: x86/xen: intercept xen hypercalls if enabled Joao Martins
2019-02-21 18:29   ` Sean Christopherson
2019-02-21 20:56     ` Joao Martins
2019-02-22  0:30       ` Sean Christopherson
2019-02-22 12:50         ` Paolo Bonzini
2020-12-01  9:48   ` David Woodhouse
2020-12-01 11:19     ` David Woodhouse
2020-12-02 11:17       ` Joao Martins
2020-12-02 12:12         ` David Woodhouse
2020-12-02  5:19     ` Ankur Arora
2020-12-02  8:03       ` David Woodhouse
2020-12-02 18:20         ` Ankur Arora
2019-02-20 20:15 ` [PATCH RFC 03/39] KVM: x86/xen: register shared_info page Joao Martins
2020-12-01 13:07   ` David Woodhouse
2020-12-02  0:40     ` Ankur Arora
2020-12-02  1:26       ` David Woodhouse
2020-12-02  5:17         ` Ankur Arora
2020-12-02 10:50           ` Joao Martins
2020-12-02 10:44       ` Joao Martins
2020-12-02 12:20         ` David Woodhouse
2020-12-02 20:32           ` Ankur Arora
2020-12-03 10:16             ` David Woodhouse
2020-12-04 17:30               ` Sean Christopherson
2020-12-02 20:33         ` Ankur Arora
2020-12-12 12:07       ` David Woodhouse
2019-02-20 20:15 ` [PATCH RFC 04/39] KVM: x86/xen: setup pvclock updates Joao Martins
2019-02-20 20:15 ` [PATCH RFC 05/39] KVM: x86/xen: update wallclock region Joao Martins
2019-02-20 20:15 ` [PATCH RFC 06/39] KVM: x86/xen: register vcpu info Joao Martins
2019-02-20 20:15 ` [PATCH RFC 07/39] KVM: x86/xen: register vcpu time info region Joao Martins
2019-02-20 20:15 ` [PATCH RFC 08/39] KVM: x86/xen: register steal clock Joao Martins
2019-02-20 20:15 ` [PATCH RFC 09/39] KVM: x86: declare Xen HVM guest capability Joao Martins
2019-02-20 20:15 ` [PATCH RFC 10/39] KVM: x86/xen: support upcall vector Joao Martins
2020-12-02 11:17   ` David Woodhouse
2020-12-02 13:12     ` Joao Martins
2020-12-02 16:47       ` David Woodhouse
2020-12-02 18:34         ` Joao Martins
2020-12-02 19:02           ` David Woodhouse
2020-12-02 20:12             ` Joao Martins
2020-12-02 20:37               ` David Woodhouse
2020-12-03  1:08             ` Ankur Arora
2020-12-08 16:08             ` David Woodhouse
2020-12-09  6:35               ` Ankur Arora
2020-12-09 10:27                 ` David Woodhouse
2020-12-09 10:51                   ` Joao Martins
2020-12-09 11:39                     ` David Woodhouse
2020-12-09 13:26                       ` Joao Martins
2020-12-09 15:41                         ` David Woodhouse
2020-12-09 16:12                           ` Joao Martins
2021-01-01 14:33           ` David Woodhouse
2021-01-05 12:11             ` Joao Martins
2021-01-05 13:23               ` David Woodhouse
2019-02-20 20:15 ` [PATCH RFC 11/39] KVM: x86/xen: evtchn signaling via eventfd Joao Martins
2020-11-30  9:41   ` David Woodhouse
2020-11-30 12:17     ` Joao Martins
2020-11-30 12:55       ` David Woodhouse
2020-11-30 15:08         ` Joao Martins
2020-11-30 16:48           ` David Woodhouse
2020-11-30 17:15             ` Joao Martins
2020-11-30 18:01               ` David Woodhouse
2020-11-30 18:41                 ` Joao Martins
2020-11-30 19:04                   ` David Woodhouse
2020-11-30 19:25                     ` Joao Martins
2021-11-23 13:15           ` David Woodhouse
2019-02-20 20:15 ` [PATCH RFC 12/39] KVM: x86/xen: store virq when assigning evtchn Joao Martins
     [not found]   ` <b750291466f3c89e0a393e48079c087704b217a5.camel@amazon.co.uk>
2022-02-10 12:17     ` Joao Martins
2022-02-10 15:23       ` [EXTERNAL] " David Woodhouse
2019-02-20 20:15 ` [PATCH RFC 13/39] KVM: x86/xen: handle PV timers oneshot mode Joao Martins
2019-02-20 20:15 ` [PATCH RFC 14/39] KVM: x86/xen: handle PV IPI vcpu yield Joao Martins
2019-02-20 20:15 ` [PATCH RFC 15/39] KVM: x86/xen: handle PV spinlocks slowpath Joao Martins
2022-02-08 12:36   ` David Woodhouse
2022-02-10 12:17     ` Joao Martins
2022-02-10 14:11       ` David Woodhouse
2019-02-20 20:15 ` [PATCH RFC 16/39] KVM: x86: declare Xen HVM evtchn offload capability Joao Martins
2019-02-20 20:15 ` [PATCH RFC 17/39] x86/xen: export vcpu_info and shared_info Joao Martins
2019-02-20 20:15 ` [PATCH RFC 18/39] x86/xen: make hypercall_page generic Joao Martins
2019-02-20 20:15 ` [PATCH RFC 19/39] xen/xenbus: xenbus uninit support Joao Martins
2019-02-20 20:15 ` [PATCH RFC 20/39] xen-blkback: module_exit support Joao Martins
2019-02-25 18:57   ` Konrad Rzeszutek Wilk
2019-02-26 11:20     ` Joao Martins
2019-02-20 20:15 ` [PATCH RFC 21/39] KVM: x86/xen: domid allocation Joao Martins
2019-02-20 20:15 ` [PATCH RFC 22/39] KVM: x86/xen: grant table init Joao Martins
2019-02-20 20:15 ` [PATCH RFC 23/39] KVM: x86/xen: grant table grow support Joao Martins
2019-02-20 20:15 ` [PATCH RFC 24/39] KVM: x86/xen: backend hypercall support Joao Martins
2019-02-20 20:15 ` [PATCH RFC 25/39] KVM: x86/xen: grant map support Joao Martins
2019-02-20 20:15 ` [PATCH RFC 26/39] KVM: x86/xen: grant unmap support Joao Martins
2019-02-20 20:15 ` [PATCH RFC 27/39] KVM: x86/xen: grant copy support Joao Martins
2019-02-20 20:15 ` [PATCH RFC 28/39] KVM: x86/xen: interdomain evtchn support Joao Martins
2019-02-20 20:15 ` [PATCH RFC 29/39] KVM: x86/xen: evtchn unmask support Joao Martins
2019-02-20 20:16 ` [PATCH RFC 30/39] KVM: x86/xen: add additional evtchn ops Joao Martins
2019-02-20 20:16 ` [PATCH RFC 31/39] xen-shim: introduce shim domain driver Joao Martins
2019-02-20 20:16 ` [PATCH RFC 32/39] xen/balloon: xen_shim_domain() support Joao Martins
2019-02-20 20:16 ` [PATCH RFC 33/39] xen/grant-table: " Joao Martins
2019-02-20 20:16 ` [PATCH RFC 34/39] xen/gntdev: " Joao Martins
2019-02-20 20:16 ` [PATCH RFC 35/39] xen/xenbus: " Joao Martins
2019-02-20 20:16 ` [PATCH RFC 36/39] drivers/xen: " Joao Martins
2019-02-20 20:16 ` [PATCH RFC 37/39] xen-netback: " Joao Martins
2019-02-20 20:16 ` [PATCH RFC 38/39] xen-blkback: " Joao Martins
2019-02-20 20:16 ` [PATCH RFC 39/39] KVM: x86: declare Xen HVM Dom0 capability Joao Martins
2019-02-20 21:09 ` [PATCH RFC 00/39] x86/KVM: Xen HVM guest support Paolo Bonzini
2019-02-21  0:29   ` Ankur Arora
2019-02-21 11:45   ` Joao Martins
2019-02-22 16:59     ` Paolo Bonzini
2019-03-12 17:14       ` Joao Martins
2019-04-08  6:44         ` Juergen Gross
2019-04-08 10:36           ` Joao Martins
2019-04-08 10:42             ` Juergen Gross
2019-04-08 17:31               ` Joao Martins
2019-04-09  0:35                 ` Stefano Stabellini
2019-04-10  5:50                   ` [Xen-devel] " Ankur Arora
2019-04-10 20:45                     ` Stefano Stabellini
2019-04-09  5:04                 ` Juergen Gross
2019-04-10  6:55                   ` Ankur Arora
2019-04-10  7:14                     ` Juergen Gross
2019-02-20 23:39 ` [Xen-devel] " Marek Marczykowski-Górecki
2019-02-21  0:31   ` Ankur Arora
2019-02-21  7:57   ` Juergen Gross
2019-02-21 12:00     ` Joao Martins
2019-02-21 11:55   ` Joao Martins

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