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Date:   Mon, 7 Aug 2023 08:55:05 +0200
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Subject: Re: [PATCH v5 04/19] KVM:x86: Refresh CPUID on write to guest
 MSR_IA32_XSS
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To:     Sean Christopherson <seanjc@google.com>,
        Yang Weijiang <weijiang.yang@intel.com>
Cc:     peterz@infradead.org, john.allen@amd.com, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org, rick.p.edgecombe@intel.com,
        chao.gao@intel.com, binbin.wu@linux.intel.com,
        Zhang Yi Z <yi.z.zhang@linux.intel.com>
References: <20230803042732.88515-1-weijiang.yang@intel.com>
 <20230803042732.88515-5-weijiang.yang@intel.com>
 <ZM1C+ILRMCfzJxx7@google.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
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On 8/4/23 20:27, Sean Christopherson wrote:
> I think my preference is to enforce guest CPUID for host accesses to 
> XSS, XFD, XFD_ERR, etc 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.

Fortunately, you are right now.  Well, almost :) but the important part 
is that indeed the dependencies are too complex.

While host-side accesses must be allowed, they should only allow the 
default value if the CPUID bit is not set.

Paolo