From: Sean Christopherson <seanjc@google.com>
To: Igor Mammedov <imammedo@redhat.com>
Cc: Vitaly Kuznetsov <vkuznets@redhat.com>,
kvm@vger.kernel.org, Paolo Bonzini <pbonzini@redhat.com>,
Wanpeng Li <wanpengli@tencent.com>,
Jim Mattson <jmattson@google.com>,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH 0/5] KVM: x86: Partially allow KVM_SET_CPUID{,2} after KVM_RUN for CPU hotplug
Date: Fri, 14 Jan 2022 16:21:57 +0000 [thread overview]
Message-ID: <YeGjJRfpJ8mMa6c7@google.com> (raw)
In-Reply-To: <20220114100849.277c04ee@redhat.com>
On Fri, Jan 14, 2022, Igor Mammedov wrote:
> On Thu, 13 Jan 2022 20:00:08 +0000
> Sean Christopherson <seanjc@google.com> wrote:
>
> > On Thu, Jan 13, 2022, Vitaly Kuznetsov wrote:
> > > 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 allowlist
> > > of entries which are allowed to change.
> >
> > Honestly, I'd prefer we give up and just revert feb627e8d6f6 ("KVM: x86: Forbid
> > KVM_SET_CPUID{,2} after KVM_RUN"). Attempting to retroactively restrict the
> > existing ioctls is becoming a mess, and I'm more than a bit concerned that this
> > will be a maintenance nightmare in the future, without all that much benefit to
> > anyone.
>
> in 63f5a1909f9 ("KVM: x86: Alert userspace that KVM_SET_CPUID{,2} after KVM_RUN is broken")
> you mention heterogeneous configuration, and that implies that
> a userspace (not upstream qemu today) might attempt to change CPUID
> and that would be wrong. Do we still care about that?
We still care, and I really do like the idea in theory, but if we're stuck choosing
between taking on a pile of ugly code in KVM and letting userspace shoot themselves
in the foot, I choose the latter :-)
prev parent reply other threads:[~2022-01-14 16:22 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-01-13 13:36 [PATCH 0/5] KVM: x86: Partially allow KVM_SET_CPUID{,2} after KVM_RUN for CPU hotplug Vitaly Kuznetsov
2022-01-13 13:36 ` [PATCH 1/5] KVM: x86: Fix indentation in kvm_set_cpuid() Vitaly Kuznetsov
2022-01-13 13:37 ` [PATCH 2/5] KVM: x86: Do runtime CPUID update before updating vcpu->arch.cpuid_entries Vitaly Kuznetsov
2022-01-13 13:37 ` [PATCH 3/5] KVM: x86: Partially allow KVM_SET_CPUID{,2} after KVM_RUN Vitaly Kuznetsov
2022-01-13 13:37 ` [PATCH 4/5] KVM: selftests: Rename 'get_cpuid_test' to 'cpuid_test' Vitaly Kuznetsov
2022-01-13 13:37 ` [PATCH 5/5] KVM: selftests: Test KVM_SET_CPUID2 after KVM_RUN Vitaly Kuznetsov
2022-01-13 20:00 ` [PATCH 0/5] KVM: x86: Partially allow KVM_SET_CPUID{,2} after KVM_RUN for CPU hotplug Sean Christopherson
2022-01-14 8:12 ` Vitaly Kuznetsov
2022-01-14 9:08 ` Igor Mammedov
2022-01-14 16:21 ` Sean Christopherson [this message]
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