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Subject: KVM_SET_CPUID doesn't check supported bits (was Re: [PATCH 0/6] KVM:
 x86: KVM_SET_SREGS.CR4 bug fixes and cleanup)
To:     Sean Christopherson <sean.j.christopherson@intel.com>
Cc:     Paolo Bonzini <pbonzini@redhat.com>,
        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
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 <20201009040453.GA10744@linux.intel.com>
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From:   stsp <stsp2@yandex.ru>
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Date:   Mon, 7 Dec 2020 14:19:37 +0300
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09.10.2020 18:30, Sean Christopherson пишет:
>> The only other effect of setting VMXE was clearing VME. Which shouldn't
>> affect anything either, right?
> Hmm, clearing VME would mean that exceptions/interrupts within the guest would
> trigger a switch out of v86 and into vanilla protected mode.  v86 and PM have
> different consistency checks, particularly for segmentation, so it's plausible
> that clearing CR4.VME inadvertantly worked around the bug by avoiding invalid
> guest state for v86.

Almost.

So with your patch set (thanks!) and a
bit of further investigations, it now became
clear where the problem is.
We have this code:
---

|cpuid->nent = 2; // Use the same values as in emu-i386/simx86/interp.c 
// (Pentium 133-200MHz, "GenuineIntel") cpuid->entries[0] = (struct 
kvm_cpuid_entry) { .function = 0, .eax = 1, .ebx = 0x756e6547, .ecx = 
0x6c65746e, .edx = 0x49656e69 }; // family 5, model 2, stepping 12, fpu 
vme de pse tsc msr mce cx8 cpuid->entries[1] = (struct kvm_cpuid_entry) 
{ .function = 1, .eax = 0x052c, .ebx = 0, .ecx = 0, .edx = 0x1bf }; ret 
= ioctl(vcpufd, KVM_SET_CPUID, cpuid); free(cpuid); if (ret == -1) { 
perror("KVM: KVM_SET_CPUID"); return 0; } --- 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?
||