linux-kernel - Re: [RFC PATCH 08/21] KVM: x86: Add kvm_x86

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Date:   Sat, 27 Jul 2019 10:38:03 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        X86 ML <x86@...nel.org>,
        Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        Joerg Roedel <joro@...tes.org>,
        "H. Peter Anvin" <hpa@...or.com>, kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, linux-sgx@...r.kernel.org
Subject: Re: [RFC PATCH 08/21] KVM: x86: Add kvm_x86_ops hook to short circuit emulation

On Fri, Jul 26, 2019 at 10:52 PM Sean Christopherson
<sean.j.christopherson@...el.com> wrote:
>
> 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.
>
> 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.
>
> 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().
>

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?

Also, anyone want to fix that emulate_ud() bug?  The test case is merged now:

# ./tools/testing/selftests/x86/syscall_arg_fault_32
[RUN]    SYSENTER with invalid state
[OK]    Seems okay
[RUN]    SYSCALL with invalid state
[SKIP]    Illegal instruction
[RUN]    SYSENTER with TF and invalid state
[OK]    Seems okay
[RUN]    SYSCALL with TF and invalid state
[WARN]    Got stuck single-stepping -- you probably have a KVM bug