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From: Sean Christopherson <seanjc@google.com>
To: Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: kvm@vger.kernel.org, Paolo Bonzini <pbonzini@redhat.com>,
	Anirudh Rayabharam <anrayabh@linux.microsoft.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Jim Mattson <jmattson@google.com>,
	Maxim Levitsky <mlevitsk@redhat.com>,
	linux-hyperv@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 06/28] KVM: nVMX: Introduce KVM_CAP_HYPERV_ENLIGHTENED_VMCS2
Date: Wed, 6 Jul 2022 21:35:56 +0000	[thread overview]
Message-ID: <YsYAPL1UUKJB3/MJ@google.com> (raw)
In-Reply-To: <20220629150625.238286-7-vkuznets@redhat.com>

On Wed, Jun 29, 2022, Vitaly Kuznetsov wrote:
> Turns out Enlightened VMCS can gain new fields without version change
> and KVM_CAP_HYPERV_ENLIGHTENED_VMCS which KVM currently has cant's
> handle this reliably. In particular, just updating the current definition
> of eVMCSv1 with the new fields and adjusting the VMX MSR filtering will
> inevitably break live migration to older KVMs. Note: enabling eVMCS and
> setting VMX feature MSR can happen in any order.
> 
> Introduce a notion of KVM internal "Enlightened VMCS revision" and add
> a new capability allowing to add fields to Enlightened VMCS while keeping
> its version.

Bumping a "minor" version number in KVM is going to be a nightmare.  KVM is going
to be stuck "supporting" old revisions in perpetuity, and userspace will be forced
to keep track of which features are available with which arbitrary revision (is
that information even communicated to userspace?).

I think a more maintainable approach would be to expose the "filtered" VMX MSRs to
userspace, e.g. add KVM_GET_EVMCS_VMX_MSRS.  Then KVM just needs to document what
the "filters" are for KVM versions that don't support KVM_GET_EVMCS_VMX_MSRS.
KVM itself doesn't need to maintain version information because it's userspace's
responsibility to ensure that userspace doesn't try to migrate to a KVM that doesn't
support the desired feature set.

That also avoids messes like unnecessarily blocking migration from "incompatible"
revisions when running on hardware that doesn't even support the control.

  reply	other threads:[~2022-07-06 21:36 UTC|newest]

Thread overview: 62+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-06-29 15:05 [PATCH v2 00/28] KVM: VMX: Support TscScaling and EnclsExitingBitmap with eVMCS + use vmcs_config for L1 VMX MSRs Vitaly Kuznetsov
2022-06-29 15:05 ` [PATCH v2 01/28] KVM: x86: hyper-v: Expose access to debug MSRs in the partition privilege flags Vitaly Kuznetsov
2022-06-29 15:05 ` [PATCH v2 02/28] x86/hyperv: Fix 'struct hv_enlightened_vmcs' definition Vitaly Kuznetsov
2022-06-29 15:06 ` [PATCH v2 03/28] x86/hyperv: Update " Vitaly Kuznetsov
2022-06-29 15:06 ` [PATCH v2 04/28] KVM: VMX: Define VMCS-to-EVMCS conversion for the new fields Vitaly Kuznetsov
2022-06-29 15:06 ` [PATCH v2 05/28] KVM: nVMX: Support several new fields in eVMCSv1 Vitaly Kuznetsov
2022-06-29 15:06 ` [PATCH v2 06/28] KVM: nVMX: Introduce KVM_CAP_HYPERV_ENLIGHTENED_VMCS2 Vitaly Kuznetsov
2022-07-06 21:35   ` Sean Christopherson [this message]
2022-07-07  9:58     ` Vitaly Kuznetsov
2022-07-07 16:17       ` Sean Christopherson
2022-07-07 16:40         ` Sean Christopherson
2022-06-29 15:06 ` [PATCH v2 07/28] KVM: selftests: Switch to KVM_CAP_HYPERV_ENLIGHTENED_VMCS2 Vitaly Kuznetsov
2022-06-29 15:06 ` [PATCH v2 08/28] KVM: VMX: Support TSC scaling with enlightened VMCS Vitaly Kuznetsov
2022-07-07 10:01   ` Vitaly Kuznetsov
2022-06-29 15:06 ` [PATCH v2 09/28] KVM: selftests: Add ENCLS_EXITING_BITMAP{,HIGH} VMCS fields Vitaly Kuznetsov
2022-06-29 15:06 ` [PATCH v2 10/28] KVM: selftests: Switch to updated eVMCSv1 definition Vitaly Kuznetsov
2022-06-29 15:06 ` [PATCH v2 11/28] KVM: selftests: Enable TSC scaling in evmcs selftest Vitaly Kuznetsov
2022-06-29 15:06 ` [PATCH v2 12/28] KVM: VMX: Enable VM_{EXIT,ENTRY}_LOAD_IA32_PERF_GLOBAL_CTRL for KVM on Hyper-V Vitaly Kuznetsov
2022-06-29 15:06 ` [PATCH v2 13/28] KVM: VMX: Get rid of eVMCS specific VMX controls sanitization Vitaly Kuznetsov
2022-07-06 22:27   ` Sean Christopherson
2022-07-07 10:03     ` Vitaly Kuznetsov
2022-06-29 15:06 ` [PATCH v2 14/28] KVM: VMX: Check VM_ENTRY_IA32E_MODE in setup_vmcs_config() Vitaly Kuznetsov
2022-06-29 20:28   ` Jim Mattson
2022-06-29 15:06 ` [PATCH v2 15/28] KVM: VMX: Check CPU_BASED_{INTR,NMI}_WINDOW_EXITING " Vitaly Kuznetsov
2022-07-01  3:58   ` Jim Mattson
2022-07-01  8:12     ` Vitaly Kuznetsov
2022-07-06 18:44   ` Sean Christopherson
2022-06-29 15:06 ` [PATCH v2 16/28] KVM: VMX: Tweak the special handling of SECONDARY_EXEC_ENCLS_EXITING " Vitaly Kuznetsov
2022-06-29 18:56   ` Jim Mattson
2022-06-30  7:32     ` Vitaly Kuznetsov
2022-06-29 15:06 ` [PATCH v2 17/28] KVM: VMX: Extend VMX controls macro shenanigans Vitaly Kuznetsov
2022-06-29 15:06 ` [PATCH v2 18/28] KVM: VMX: Move CPU_BASED_CR8_{LOAD,STORE}_EXITING filtering out of setup_vmcs_config() Vitaly Kuznetsov
2022-07-01  4:14   ` Jim Mattson
2022-06-29 15:06 ` [PATCH v2 19/28] KVM: VMX: Add missing VMEXIT controls to vmcs_config Vitaly Kuznetsov
2022-07-01 16:02   ` Jim Mattson
2022-06-29 15:06 ` [PATCH v2 20/28] KVM: VMX: Add missing VMENTRY " Vitaly Kuznetsov
2022-07-01 16:01   ` Jim Mattson
2022-07-07 10:41     ` Vitaly Kuznetsov
2022-06-29 15:06 ` [PATCH v2 21/28] KVM: VMX: Add missing CPU based VM execution " Vitaly Kuznetsov
2022-07-01 16:05   ` Jim Mattson
2022-06-29 15:06 ` [PATCH v2 22/28] KVM: VMX: Clear controls obsoleted by EPT at runtime, not setup Vitaly Kuznetsov
2022-07-01 16:11   ` Jim Mattson
2022-07-07 14:57     ` Vitaly Kuznetsov
2022-07-07 19:30       ` Sean Christopherson
2022-07-07 21:32         ` Jim Mattson
2022-07-07 21:39           ` Sean Christopherson
2022-07-07 23:12             ` Jim Mattson
2022-07-08  7:55             ` Vitaly Kuznetsov
2022-06-29 15:06 ` [PATCH v2 23/28] KVM: VMX: Move LOAD_IA32_PERF_GLOBAL_CTRL errata handling out of setup_vmcs_config() Vitaly Kuznetsov
2022-07-01 16:26   ` Jim Mattson
2022-07-07 12:07     ` Vitaly Kuznetsov
2022-06-29 15:06 ` [PATCH v2 24/28] KVM: nVMX: Use sanitized allowed-1 bits for VMX control MSRs Vitaly Kuznetsov
2022-07-01 22:54   ` Jim Mattson
2022-07-07 12:30     ` Vitaly Kuznetsov
2022-06-29 15:06 ` [PATCH v2 25/28] KVM: VMX: Store required-1 VMX controls in vmcs_config Vitaly Kuznetsov
2022-06-29 15:06 ` [PATCH v2 26/28] KVM: nVMX: Use sanitized required-1 bits for VMX control MSRs Vitaly Kuznetsov
2022-06-29 15:06 ` [PATCH v2 27/28] KVM: VMX: Cache MSR_IA32_VMX_MISC in vmcs_config Vitaly Kuznetsov
2022-06-29 18:33   ` Jim Mattson
2022-06-29 15:06 ` [PATCH v2 28/28] KVM: nVMX: Use cached host MSR_IA32_VMX_MISC value for setting up nested MSR Vitaly Kuznetsov
2022-06-29 18:37   ` Jim Mattson
2022-06-30  7:36     ` Vitaly Kuznetsov
2022-06-30 12:27       ` Jim Mattson

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