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

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Message-ID: <3d827e8b-a04e-0a93-4bb4-e0e9d59036da@redhat.com>
Date:   Wed, 6 Nov 2019 13:17:40 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Alexander Graf <graf@...zon.com>
Cc:     Radim Krčmář <rkrcmar@...hat.com>,
        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 06/11/19 01:58, Sean Christopherson wrote:
>> 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.
> Hmm, I think it'd make more sense to use #define instead of enum to
> hopefully make it clear that they aren't the *only* values that can be
> returned.  That'd also prevent anyone from changing the return types from
> 'int' to 'enum kvm_return', which IMO would hurt readability overall.
> 
> And maybe KVM_EXIT_TO_USERSPACE and KVM_RETURN_TO_GUEST?

That would be quite some work.  Right now there is some consistency
between all of:

- x86_emulate_instruction and its callers

- vcpu->arch.complete_userspace_io

- vcpu_enter_guest/vcpu_block

- kvm_x86_ops->handle_exit

so it would be very easy to end up with a half-int-half-enum state that
is more confusing than before...

I'm more worried about cases where we have functions returning either 0
or -errno, but 0 lets you enter the guest.  I'm not sure if the only one
is kvm_mmu_reload or there are others.

Paolo