linux-kernel - Re: [PATCH v2 00/14] KVM: x86: Remove emulation

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Message-ID: <686b499e-7700-228e-3602-8e0979177acb@amazon.com>
Date:   Fri, 25 Oct 2019 13:00:03 +0200
From:   Alexander Graf <graf@...zon.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Radim Krčmář <rkrcmar@...hat.com>
CC:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, <kvm@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, Liran Alon <liran.alon@...cle.com>
Subject: Re: [PATCH v2 00/14] KVM: x86: Remove emulation_result enums

On 17.09.19 17:14, Paolo Bonzini wrote:
> On 27/08/19 23:40, Sean Christopherson wrote:
>> Rework the emulator and its users to handle failure scenarios entirely
>> within the emulator.
>>
>> {x86,kvm}_emulate_instruction() currently returns a tri-state value to
>> indicate success/continue, userspace exit needed, and failure.  The
>> intent of returning EMULATE_FAIL is to let the caller handle failure in
>> a manner that is appropriate for the current context.  In practice,
>> the emulator has ended up with a mixture of failure handling, i.e.
>> whether or not the emulator takes action on failure is dependent on the
>> specific flavor of emulation.
>>
>> The mixed handling has proven to be rather fragile, e.g. many flows
>> incorrectly assume their specific flavor of emulation cannot fail or
>> that the emulator sets state to report the failure back to userspace.
>>
>> Move everything inside the emulator, piece by piece, so that the
>> emulation routines can return '0' for exit to userspace and '1' for
>> resume the guest, just like every other VM-Exit handler.
>>
>> Patch 13/14 is a tangentially related bug fix that conflicts heavily with
>> this series, so I tacked it on here.
>>
>> Patch 14/14 documents the emulation types.  I added it as a separate
>> patch at the very end so that the comments could reference the final
>> state of the code base, e.g. incorporate the rule change for using
>> EMULTYPE_SKIP that is introduced in patch 13/14.
>>
>> v1:
>>    - https://patchwork.kernel.org/cover/11110331/
>>
>> v2:
>>    - Collect reviews. [Vitaly and Liran]
>>    - Squash VMware emultype changes into a single patch. [Liran]
>>    - Add comments in VMX/SVM for VMware #GP handling. [Vitaly]
>>    - Tack on the EPT misconfig bug fix.
>>    - Add a patch to comment/document the emultypes. [Liran]
>>
>> Sean Christopherson (14):
>>    KVM: x86: Relocate MMIO exit stats counting
>>    KVM: x86: Clean up handle_emulation_failure()
>>    KVM: x86: Refactor kvm_vcpu_do_singlestep() to remove out param
>>    KVM: x86: Don't attempt VMWare emulation on #GP with non-zero error
>>      code
>>    KVM: x86: Move #GP injection for VMware into x86_emulate_instruction()
>>    KVM: x86: Add explicit flag for forced emulation on #UD
>>    KVM: x86: Move #UD injection for failed emulation into emulation code
>>    KVM: x86: Exit to userspace on emulation skip failure
>>    KVM: x86: Handle emulation failure directly in kvm_task_switch()
>>    KVM: x86: Move triple fault request into RM int injection
>>    KVM: VMX: Remove EMULATE_FAIL handling in handle_invalid_guest_state()
>>    KVM: x86: Remove emulation_result enums, EMULATE_{DONE,FAIL,USER_EXIT}
>>    KVM: VMX: Handle single-step #DB for EMULTYPE_SKIP on EPT misconfig
>>    KVM: x86: Add comments to document various emulation types
>>
>>   arch/x86/include/asm/kvm_host.h |  40 +++++++--
>>   arch/x86/kvm/mmu.c              |  16 +---
>>   arch/x86/kvm/svm.c              |  62 ++++++--------
>>   arch/x86/kvm/vmx/vmx.c          | 147 +++++++++++++-------------------
>>   arch/x86/kvm/x86.c              | 133 ++++++++++++++++-------------
>>   arch/x86/kvm/x86.h              |   2 +-
>>   6 files changed, 195 insertions(+), 205 deletions(-)
>>
> 
> Queued, thanks (a couple conflicts had to be sorted out, but nothing
> requiring a respin).

Ugh, I just stumbled over this commit. Is this really the right 
direction to move towards?

I appreciate the move to reduce the emulator logic from the many-fold 
enum into a simple binary "worked" or "needs a user space exit". But are 
"0" and "1" really the right names for that? I find the readability of 
the current intercept handlers bad enough, trickling that into even more 
code sounds like a situation that will decrease readability even more.

Why can't we just use names throughout? Something like

enum kvm_return {
     KVM_RET_USER_EXIT = 0,
     KVM_RET_GUEST = 1,
};

and then consistently use them as return values? That way anyone who has 
not worked on kvm before can still make sense of the code.


Alex



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