Re: [PATCH v2 0/4] KVM: x86: Partially allow KVM_SET_CPUID{,2} after KVM_RUN for CPU hotplug

From: Igor Mammedov <imammedo@redhat.com>
To: Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: kvm@vger.kernel.org, Paolo Bonzini <pbonzini@redhat.com>,
	Sean Christopherson <seanjc@google.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Jim Mattson <jmattson@google.com>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 0/4] KVM: x86: Partially allow KVM_SET_CPUID{,2} after KVM_RUN for CPU hotplug
Date: Tue, 18 Jan 2022 15:35:31 +0100	[thread overview]
Message-ID: <20220118153531.11e73048@redhat.com> (raw)
In-Reply-To: <20220117150542.2176196-1-vkuznets@redhat.com>

On Mon, 17 Jan 2022 16:05:38 +0100
Vitaly Kuznetsov <vkuznets@redhat.com> wrote:

> Changes since v1:
> - Drop the allowlist of items which were allowed to change and just allow
> the exact same CPUID data [Sean, Paolo]. Adjust selftest accordingly.
> - Drop PATCH1 as the exact same change got merged upstream.
> 
> Recently, KVM made it illegal to change CPUID after KVM_RUN but
> unfortunately this change is not fully compatible with existing VMMs.
> In particular, QEMU reuses vCPU fds for CPU hotplug after unplug and it
> calls KVM_SET_CPUID2. Relax the requirement by implementing an allowing
> KVM_SET_CPUID{,2} with the exact same data.

Can you check following scenario:
 * on host that has IA32_TSX_CTRL and TSX enabled (RTM/HLE cpuid bits present)
 * boot 2 vcpus VM with TSX enabled on VMM side but with tsx=off on kernel CLI

     that should cause kernel to set MSR_IA32_TSX_CTRL to 3H from initial 0H
     and clear RTM+HLE bits in CPUID, check that RTM/HLE cpuid it cleared

 * hotunplug a VCPU and then replug it again
    if IA32_TSX_CTRL is reset to initial state, that should re-enable
    RTM/HLE cpuid bits and KVM_SET_CPUID2 might fail due to difference

and as Sean pointed out there might be other non constant leafs,
where exact match check could leave userspace broken.

> Vitaly Kuznetsov (4):
>   KVM: x86: Do runtime CPUID update before updating
>     vcpu->arch.cpuid_entries
>   KVM: x86: Partially allow KVM_SET_CPUID{,2} after KVM_RUN
>   KVM: selftests: Rename 'get_cpuid_test' to 'cpuid_test'
>   KVM: selftests: Test KVM_SET_CPUID2 after KVM_RUN
> 
>  arch/x86/kvm/cpuid.c                          | 67 ++++++++++++++++---
>  arch/x86/kvm/x86.c                            | 19 ------
>  tools/testing/selftests/kvm/.gitignore        |  2 +-
>  tools/testing/selftests/kvm/Makefile          |  4 +-
>  .../selftests/kvm/include/x86_64/processor.h  |  7 ++
>  .../selftests/kvm/lib/x86_64/processor.c      | 33 +++++++--
>  .../x86_64/{get_cpuid_test.c => cpuid_test.c} | 30 +++++++++
>  7 files changed, 126 insertions(+), 36 deletions(-)
>  rename tools/testing/selftests/kvm/x86_64/{get_cpuid_test.c => cpuid_test.c} (83%)
> 