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Subject: Re: [RFC PATCH 08/21] KVM: x86: Add kvm_x86_ops hook to short circuit emulation
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <20190730024940.GL21120@linux.intel.com>
Date:   Thu, 15 Aug 2019 17:47:12 -0700
Cc:     Andy Lutomirski <luto@kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        =?utf-8?Q?Radim_Kr=C4=8Dm=C3=A1=C5=99?= <rkrcmar@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        X86 ML <x86@kernel.org>,
        Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>,
        Joerg Roedel <joro@8bytes.org>,
        "H. Peter Anvin" <hpa@zytor.com>, kvm list <kvm@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>, linux-sgx@vger.kernel.org
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Message-Id: <25BBDA64-1253-4429-95AF-5D578684F6CC@amacapital.net>
References: <20190727055214.9282-1-sean.j.christopherson@intel.com> <20190727055214.9282-9-sean.j.christopherson@intel.com> <CALCETrU_51Ae=F9HzUwsUuSkJ1or63p_eG+f3uKkBqFx=bheUA@mail.gmail.com> <20190730024940.GL21120@linux.intel.com>
To:     Sean Christopherson <sean.j.christopherson@intel.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


>> On Jul 29, 2019, at 7:49 PM, Sean Christopherson <sean.j.christopherson@i=
ntel.com> wrote:
>>=20
>> On Sat, Jul 27, 2019 at 10:38:03AM -0700, Andy Lutomirski wrote:
>> On Fri, Jul 26, 2019 at 10:52 PM Sean Christopherson
>> <sean.j.christopherson@intel.com> wrote:
>>>=20
>>> Similar to the existing AMD #NPF case where emulation of the current
>>> instruction is not possible due to lack of information, virtualization
>>> of Intel SGX will introduce a scenario where emulation is not possible
>>> due to the VMExit occurring in an SGX enclave.  And again similar to
>>> the AMD case, emulation can be initiated by kvm_mmu_page_fault(), i.e.
>>> outside of the control of the vendor-specific code.
>>>=20
>>> While the cause and architecturally visible behavior of the two cases
>>> is different,  e.g. Intel SGX will inject a #UD whereas AMD #NPF is a
>>> clean resume or complete shutdown, the impact on the common emulation
>>> code is identical: KVM must stop emulation immediately and resume the
>>> guest.
>>>=20
>>> Replace the exisiting need_emulation_on_page_fault() with a more generic=

>>> is_emulatable() kvm_x86_ops callback, which is called unconditionally
>>> by x86_emulate_instruction().
>>=20
>> Having recently noticed that emulate_ud() is broken when the guest's
>> TF is set, I suppose I should ask: does your new code function
>> sensibly when TF is set?
>=20
> Barring a VMX fault injection interaction I'm not thinking of, yes.  The
> SGX reaction to the #UD VM-Exit is to inject a #UD and resume the guest,
> pending breakpoints shouldn't be affected in any way (unless some other
> part of KVM mucks with them, e.g. when guest single-stepping is enabled).

What I mean is: does the code actually do what you think it does if TF is se=
t?  Right now, as I understand it, the KVM emulation code has a bug in which=
 some emulated faults also inject #DB despite the fact that the instruction f=
aulted, and the #DB seems to take precedence over the original fault.  This c=
onfuses the guest.=