linux-kernel - Re: [PATCH v5 04/19] KVM:x86: Refresh CPUID on write to guest MSR_IA32

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Date:   Thu, 10 Aug 2023 02:01:29 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     "Yang, Weijiang" <weijiang.yang@...el.com>
Cc:     Sean Christopherson <seanjc@...gle.com>, peterz@...radead.org,
        john.allen@....com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, rick.p.edgecombe@...el.com,
        chao.gao@...el.com, binbin.wu@...ux.intel.com,
        Zhang Yi Z <yi.z.zhang@...ux.intel.com>
Subject: Re: [PATCH v5 04/19] KVM:x86: Refresh CPUID on write to guest MSR_IA32_XSS

On Wed, Aug 9, 2023 at 10:56 AM Yang, Weijiang <weijiang.yang@...el.com> wrote:
> > I'm pretty sure I've advocated for the exact opposite in the past, i.e. argued
> > that KVM's ABI is to not enforce ordering between KVM_SET_CPUID2 and KVM_SET_MSR.
> > But this is becoming untenable, juggling the dependencies in KVM is complex and
> > is going to result in a nasty bug at some point.
> >
> > For this series, lets just tighten the rules for XSS, i.e. drop the host_initated
> > exemption.  And in a parallel/separate series, try to do a wholesale cleanup of
> > all the cases that essentially allow userspace to do KVM_SET_MSR before KVM_SET_CPUID2.
> OK, will do it for this series and investigate for other MSRs.
> Thanks!

Remember that, while the ordering between KVM_SET_CPUID2 and
KVM_SET_MSR must be enforced(*), the host_initiated path must allow
the default (generally 0) value.

Paolo

(*) this means that you should check guest_cpuid_has even if
host_initiated == true.