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Re: [PATCH] kvm: x86: disable KVM_FAST_MMIO_BUS

From David Hildenbrand <david@redhat.com>
Newsgroups linux.kernel
Subject Re: [PATCH] kvm: x86: disable KVM_FAST_MMIO_BUS
Date 2017-08-17 10:20 +0200
Message-ID <ufoc1-29K-17@gated-at.bofh.it> (permalink)
References <uf4Gm-6q1-43@gated-at.bofh.it> <uf5j5-6Uh-41@gated-at.bofh.it>
Organization Red Hat GmbH

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On 16.08.2017 14:07, Radim Krčmář wrote:
> 2017-08-16 13:22+0200, Paolo Bonzini:
>> Microsoft pointed out privately to me that KVM's handling of
>> KVM_FAST_MMIO_BUS is invalid.  Using skip_emulation_instruction is invalid
>> in EPT misconfiguration vmexit handlers, because neither EPT violations
>> nor misconfigurations are listed in the manual among the VM exits that
>> set the VM-exit instruction length field.
>>
>> While physical processors seem to set the field, this is not architectural
>> and is just a side effect of the implementation.  I couldn't convince
>> myself of any condition on the exit qualification where VM-exit
>> instruction length "has" to be defined; there are no trap-like VM-exits
>> that can be repurposed; and fault-like VM-exits such as descriptor-table
>> exits provide no decoding information.  So I don't really see any elegant
>> way to fix it except by disabling KVM_FAST_MMIO_BUS, which means virtio
>> 1 will go slower.
> 
> Do you have some numbers?
> 
> We could keep the ugliness in KVM and add a new skip function with
> emulate_instruction(vcpu, EMULTYPE_SKIP) to decode the length of the
> instruction.  (Adding a condition just for EPT violation exit reason to

I like that idea.

> the existing skip function would be a dirtier solution.)
> Slower than what we have now, but faster than full emulation.
> 
> I agree that configuring EPT to throw a violation when accessing fast
> MMIO has many drawbacks (although it seems to be what Intel expected).
> 
> Thanks.


-- 

Thanks,

David

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Thread

[PATCH] kvm: x86: disable KVM_FAST_MMIO_BUS Paolo Bonzini <pbonzini@redhat.com> - 2017-08-16 13:30 +0200
  Re: [PATCH] kvm: x86: disable KVM_FAST_MMIO_BUS Radim Krčmář <rkrcmar@redhat.com> - 2017-08-16 14:10 +0200
    Re: [PATCH] kvm: x86: disable KVM_FAST_MMIO_BUS Paolo Bonzini <pbonzini@redhat.com> - 2017-08-16 15:40 +0200
      Re: [PATCH] kvm: x86: disable KVM_FAST_MMIO_BUS "Michael S. Tsirkin" <mst@redhat.com> - 2017-08-16 16:10 +0200
        Re: [PATCH] kvm: x86: disable KVM_FAST_MMIO_BUS Paolo Bonzini <pbonzini@redhat.com> - 2017-08-16 16:20 +0200
    Re: [PATCH] kvm: x86: disable KVM_FAST_MMIO_BUS David Hildenbrand <david@redhat.com> - 2017-08-17 10:20 +0200
  Re: [PATCH] kvm: x86: disable KVM_FAST_MMIO_BUS "Michael S. Tsirkin" <mst@redhat.com> - 2017-08-16 15:00 +0200
    Re: [PATCH] kvm: x86: disable KVM_FAST_MMIO_BUS Paolo Bonzini <pbonzini@redhat.com> - 2017-08-16 15:10 +0200
      Re: [PATCH] kvm: x86: disable KVM_FAST_MMIO_BUS "Michael S. Tsirkin" <mst@redhat.com> - 2017-08-16 15:20 +0200
        Re: [PATCH] kvm: x86: disable KVM_FAST_MMIO_BUS Paolo Bonzini <pbonzini@redhat.com> - 2017-08-16 15:40 +0200
          Re: [PATCH] kvm: x86: disable KVM_FAST_MMIO_BUS "Michael S. Tsirkin" <mst@redhat.com> - 2017-08-16 16:10 +0200
          Re: [PATCH] kvm: x86: disable KVM_FAST_MMIO_BUS "Michael S. Tsirkin" <mst@redhat.com> - 2017-08-16 19:00 +0200
            Re: [PATCH] kvm: x86: disable KVM_FAST_MMIO_BUS Paolo Bonzini <pbonzini@redhat.com> - 2017-08-16 19:30 +0200
              Re: [PATCH] kvm: x86: disable KVM_FAST_MMIO_BUS Radim Krčmář <rkrcmar@redhat.com> - 2017-08-16 21:10 +0200
                Re: [PATCH] kvm: x86: disable KVM_FAST_MMIO_BUS "Michael S. Tsirkin" <mst@redhat.com> - 2017-08-16 22:00 +0200
                Re: [PATCH] kvm: x86: disable KVM_FAST_MMIO_BUS Paolo Bonzini <pbonzini@redhat.com> - 2017-08-16 23:30 +0200
                Re: [PATCH] kvm: x86: disable KVM_FAST_MMIO_BUS "Michael S. Tsirkin" <mst@redhat.com> - 2017-08-17 00:40 +0200
                Re: [PATCH] kvm: x86: disable KVM_FAST_MMIO_BUS Paolo Bonzini <pbonzini@redhat.com> - 2017-08-17 11:10 +0200
                Re: [PATCH] kvm: x86: disable KVM_FAST_MMIO_BUS Paolo Bonzini <pbonzini@redhat.com> - 2017-08-17 14:20 +0200
                Re: [PATCH] kvm: x86: disable KVM_FAST_MMIO_BUS Radim Krčmář <rkrcmar@redhat.com> - 2017-08-17 15:30 +0200
                Re: [PATCH] kvm: x86: disable KVM_FAST_MMIO_BUS "Michael S. Tsirkin" <mst@redhat.com> - 2017-08-17 17:20 +0200
                Re: [PATCH] kvm: x86: disable KVM_FAST_MMIO_BUS Radim Krčmář <rkrcmar@redhat.com> - 2017-08-17 16:00 +0200
                Re: [PATCH] kvm: x86: disable KVM_FAST_MMIO_BUS "Michael S. Tsirkin" <mst@redhat.com> - 2017-08-17 17:30 +0200
              Re: [PATCH] kvm: x86: disable KVM_FAST_MMIO_BUS "Michael S. Tsirkin" <mst@redhat.com> - 2017-08-16 21:50 +0200

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