linux-kernel - Re: [PATCH 00/14] KVM: nVMX: Use vmcs_config for setting up nested VMX MSRs

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Date:   Tue, 28 Jun 2022 16:04:02 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Anirudh Rayabharam <anrayabh@...ux.microsoft.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Maxim Levitsky <mlevitsk@...hat.com>,
        linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 00/14] KVM: nVMX: Use vmcs_config for setting up nested
 VMX MSRs

Jim Mattson <jmattson@...gle.com> writes:

> On Mon, Jun 27, 2022 at 9:04 AM Vitaly Kuznetsov <vkuznets@...hat.com> wrote:
>>
>> Changes since RFC:
>> - "KVM: VMX: Extend VMX controls macro shenanigans" PATCH added and the
>>   infrastructure is later used in other patches [Sean] PATCHes 1-3 added
>>   to support the change.
>> - "KVM: VMX: Clear controls obsoleted by EPT at runtime, not setup" PATCH
>>   added [Sean].
>> - Commit messages added.
>>
>> vmcs_config is a sanitized version of host VMX MSRs where some controls are
>> filtered out (e.g. when Enlightened VMCS is enabled, some know bugs are
>> discovered, some inconsistencies in controls are detected,...) but
>> nested_vmx_setup_ctls_msrs() uses raw host MSRs instead. This may end up
>> in exposing undesired controls to L1. Switch to using vmcs_config instead.
>>
>> Sean Christopherson (1):
>>   KVM: VMX: Clear controls obsoleted by EPT at runtime, not setup
>>
>> Vitaly Kuznetsov (13):
>>   KVM: VMX: Check VM_ENTRY_IA32E_MODE in setup_vmcs_config()
>>   KVM: VMX: Check CPU_BASED_{INTR,NMI}_WINDOW_EXITING in
>>     setup_vmcs_config()
>>   KVM: VMX: Tweak the special handling of SECONDARY_EXEC_ENCLS_EXITING
>>     in setup_vmcs_config()
>>   KVM: VMX: Extend VMX controls macro shenanigans
>>   KVM: VMX: Move CPU_BASED_CR8_{LOAD,STORE}_EXITING filtering out of
>>     setup_vmcs_config()
>>   KVM: VMX: Add missing VMEXIT controls to vmcs_config
>>   KVM: VMX: Add missing VMENTRY controls to vmcs_config
>>   KVM: VMX: Add missing CPU based VM execution controls to vmcs_config
>>   KVM: nVMX: Use sanitized allowed-1 bits for VMX control MSRs
>>   KVM: VMX: Store required-1 VMX controls in vmcs_config
>>   KVM: nVMX: Use sanitized required-1 bits for VMX control MSRs
>>   KVM: VMX: Cache MSR_IA32_VMX_MISC in vmcs_config
>>   KVM: nVMX: Use cached host MSR_IA32_VMX_MISC value for setting up
>>     nested MSR
>>
>>  arch/x86/kvm/vmx/capabilities.h |  16 +--
>>  arch/x86/kvm/vmx/nested.c       |  37 +++---
>>  arch/x86/kvm/vmx/nested.h       |   2 +-
>>  arch/x86/kvm/vmx/vmx.c          | 198 ++++++++++++++------------------
>>  arch/x86/kvm/vmx/vmx.h          | 118 +++++++++++++++++++
>>  5 files changed, 229 insertions(+), 142 deletions(-)
>>
>> --
>> 2.35.3
>>
>
> Just checking that this doesn't introduce any backwards-compatibility
> issues. That is, all features that were reported as being available in
> the past should still be available moving forward.
>

All the controls nested_vmx_setup_ctls_msrs() set are in the newly
introduced KVM_REQ_VMX_*/KVM_OPT_VMX_* sets so we should be good here
(unless I screwed up, of course).

There's going to be some changes though. E.g this series was started by
Anirudh's report when KVM was exposing SECONDARY_EXEC_TSC_SCALING while
running on KVM and using eVMCS which doesn't support the control. This
is a bug and I don't think we need and 'bug compatibility' here.

Another change is that VM_ENTRY_LOAD_IA32_PERF_GLOBAL_CTRL/
VM_EXIT_LOAD_IA32_PERF_GLOBAL_CTRL will now be filtered out on the
"broken" CPUs (the list is in setup_vmcs_config()). I *think* this is
also OK but if not, we can move the filtering to vmx_vmentry_ctrl()/
vmx_vmexit_ctrl().

-- 
Vitaly