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Subject: Re: [RESEND PATCH 07/13] KVM: x86: Add explicit flag for forced
 emulation on #UD
From:   Liran Alon <liran.alon@oracle.com>
In-Reply-To: <20190823144423.GB6713@linux.intel.com>
Date:   Fri, 23 Aug 2019 18:31:32 +0300
Cc:     Paolo Bonzini <pbonzini@redhat.com>,
        =?utf-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@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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Message-Id: <B03AFFBB-3E33-4D04-8B5D-4FA3C7F385CB@oracle.com>
References: <20190823010709.24879-1-sean.j.christopherson@intel.com>
 <20190823010709.24879-8-sean.j.christopherson@intel.com>
 <9E01A06E-FD3E-4D43-9FFE-6FFE3BAC269A@oracle.com>
 <20190823144423.GB6713@linux.intel.com>
To:     Sean Christopherson <sean.j.christopherson@intel.com>
Sender: linux-kernel-owner@vger.kernel.org
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> On 23 Aug 2019, at 17:44, Sean Christopherson =
<sean.j.christopherson@intel.com> wrote:
>=20
> On Fri, Aug 23, 2019 at 04:47:14PM +0300, Liran Alon wrote:
>>=20
>>=20
>>> On 23 Aug 2019, at 4:07, Sean Christopherson =
<sean.j.christopherson@intel.com> wrote:
>>>=20
>>> Add an explicit emulation type for forced #UD emulation and use it =
to
>>> detect that KVM should unconditionally inject a #UD instead of =
falling
>>> into its standard emulation failure handling.
>>>=20
>>> Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
>>=20
>> The name "forced emulation on #UD" is not clear to me.
>>=20
>> If I understand correctly, EMULTYPE_TRAP_UD is currently used to =
indicate
>> that in case the x86 emulator fails to decode instruction, the caller =
would
>> like the x86 emulator to fail early such that it can handle this =
condition
>> properly.  Thus, I would rename it EMULTYPE_TRAP_DECODE_FAILURE.
>=20
> EMULTYPE_TRAP_UD is used when KVM intercepts a #UD from hardware.  KVM
> only emulates select instructions in this case in order to minmize the
> emulator attack surface, e.g.:
>=20
> 	if (unlikely(ctxt->ud) && likely(!(ctxt->d & EmulateOnUD)))
> 		return EMULATION_FAILED;
>=20
> To enable testing of the emulator, KVM recognizes a special "opcode" =
that
> triggers full emulation on #UD, e.g. ctxt->ud is false when the #UD =
was
> triggered with the magic prefix.  The prefix is only recognized when =
the
> module param force_emulation_prefix is toggled on, hence the name
> EMULTYPE_TRAP_UD_FORCED.

Ah-ha. This makes sense. Thanks for the explanation.
I would say it=E2=80=99s worth putting a comment in it in code=E2=80=A6

>=20
>> But this new flag seems to do the same. So I=E2=80=99m left confused. =
 I=E2=80=99m probably
>> missing something trivial here.