linux-kernel - Re: [PATCH v2 00/13] KVM: x86: Allow userspace to disable the emulator

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Message-ID: <3ec358a8-859d-9ef1-7392-372d55b28ee4@redhat.com>
Date:   Mon, 2 Mar 2020 19:42:31 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.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
Subject: Re: [PATCH v2 00/13] KVM: x86: Allow userspace to disable the
 emulator

On 19/02/20 00:29, Sean Christopherson wrote:
> The primary intent of this series is to dynamically allocate the emulator
> and get KVM to a state where the emulator *could* be disabled at some
> point in the future.  Actually allowing userspace to disable the emulator
> was a minor change at that point, so I threw it in.
> 
> Dynamically allocating the emulator shrinks the size of x86 vcpus by
> ~2.5k bytes, which is important because 'struct vcpu_vmx' has once again
> fattened up and squeaked past the PAGE_ALLOC_COSTLY_ORDER threshold.
> Moving the emulator to its own allocation gives us some breathing room
> for the near future, and has some other nice side effects.
> 
> As for disabling the emulator... in the not-too-distant future, I expect
> there will be use cases that can truly disable KVM's emulator, e.g. for
> security (KVM's and/or the guests).  I don't have a strong opinion on
> whether or not KVM should actually allow userspace to disable the emulator
> without a concrete use case (unless there already is a use case?), which
> is why that part is done in its own tiny patch.
> 
> Running without an emulator has been "tested" in the sense that the
> selftests that don't require emulation continue to pass, and everything
> else fails with the expected "emulation error".

I agree with Vitaly that, if we want this, it should be a KVM_ENABLE_CAP
instead.  The first 10 patches are very nice cleanups though so I plan
to apply them (with Vitaly's suggested nits for review) after you answer
the question on patch 10.

Paolo

> 
> v2:
>   - Rebase to kvm/queue, 2c2787938512 ("KVM: selftests: Stop ...")
> 
> Sean Christopherson (13):
>   KVM: x86: Refactor I/O emulation helpers to provide vcpu-only variant
>   KVM: x86: Explicitly pass an exception struct to check_intercept
>   KVM: x86: Move emulation-only helpers to emulate.c
>   KVM: x86: Refactor R/W page helper to take the emulation context
>   KVM: x86: Refactor emulated exception injection to take the emul
>     context
>   KVM: x86: Refactor emulate tracepoint to explicitly take context
>   KVM: x86: Refactor init_emulate_ctxt() to explicitly take context
>   KVM: x86: Dynamically allocate per-vCPU emulation context
>   KVM: x86: Move kvm_emulate.h into KVM's private directory
>   KVM: x86: Shrink the usercopy region of the emulation context
>   KVM: x86: Add helper to "handle" internal emulation error
>   KVM: x86: Add variable to control existence of emulator
>   KVM: x86: Allow userspace to disable the kernel's emulator
> 
>  arch/x86/include/asm/kvm_host.h             |  12 +-
>  arch/x86/kvm/emulate.c                      |  13 +-
>  arch/x86/{include/asm => kvm}/kvm_emulate.h |   9 +-
>  arch/x86/kvm/mmu/mmu.c                      |   1 +
>  arch/x86/kvm/svm.c                          |   5 +-
>  arch/x86/kvm/trace.h                        |  22 +--
>  arch/x86/kvm/vmx/vmx.c                      |  15 +-
>  arch/x86/kvm/x86.c                          | 193 +++++++++++++-------
>  arch/x86/kvm/x86.h                          |  12 +-
>  9 files changed, 183 insertions(+), 99 deletions(-)
>  rename arch/x86/{include/asm => kvm}/kvm_emulate.h (99%)
>