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Subject: Re: KVM_SET_CPUID doesn't check supported bits (was Re: [PATCH 0/6]
 KVM: x86: KVM_SET_SREGS.CR4 bug fixes and cleanup)
To:     stsp <stsp2@yandex.ru>,
        Sean Christopherson <sean.j.christopherson@intel.com>
Cc:     Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org
References: <20201007014417.29276-1-sean.j.christopherson@intel.com>
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 <20201008175951.GA9267@linux.intel.com>
 <7efe1398-24c0-139f-29fa-3d89b6013f34@yandex.ru>
 <20201009040453.GA10744@linux.intel.com>
 <5dfa55f3-ecdf-9f8d-2d45-d2e6e54f2daa@yandex.ru>
 <20201009153053.GA16234@linux.intel.com>
 <b38dff0b-7e6d-3f3e-9724-8e280938628a@yandex.ru>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <c206865e-b2da-b996-3d48-2c71d7783fbc@redhat.com>
Date:   Mon, 7 Dec 2020 12:29:34 +0100
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On 07/12/20 12:24, stsp wrote:
> It tries to enable VME among other things.
> qemu appears to disable VME by default,
> unless you do "-cpu host". So we have a situation where
> the host (which is qemu) doesn't have VME,
> and guest (dosemu) is trying to enable it.
> Now obviously KVM_SET_CPUID doesn't check anyting
> at all and returns success. That later turns
> into an invalid guest state.
> 
> 
> Question: should KVM_SET_CPUID check for
> supported bits, end return error if not everything
> is supported?

No, it is intentional.  Most bits of CPUID are not ever checked by KVM, 
so userspace is supposed to set values that makes sense or just copy the 
value of KVM_GET_SUPPORTED_CPUID more or less blindly.

Paolo