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

Paolo Bonzini
2020-03-02T18:42:42Z

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%)
>