Re: [PATCH] kvm: VMX: do not use vm-exit instruction length for fast MMIO

[Yang Zhang]

On 2017/8/17 16:31, Wanpeng Li wrote:
> 2017-08-17 16:28 GMT+08:00 Wanpeng Li <kernellwp@xxxxxxxxx>:
> > 2017-08-17 16:07 GMT+08:00 Yang Zhang <yang.zhang.wz@xxxxxxxxx>:
> > > On 2017/8/17 0:56, Radim KrÄmÃÅ wrote:
> > > > 2017-08-16 17:10+0300, Michael S. Tsirkin:
> > > > > On Wed, Aug 16, 2017 at 03:34:54PM +0200, Paolo Bonzini wrote:
> > > > > > 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 way
> > > > > > to keep the full speedup.
> > > > > >
> > > > > > What we can do is use EMULTYPE_SKIP; it only saves 200 clock cycles
> > > > > > because computing the physical RIP and reading the instruction is
> > > > > > expensive, but at least the eventfd is signaled before entering the
> > > > > > emulator.  This saves on latency.  While at it, don't check breakpoints
> > > > > > when skipping the instruction, as presumably any side effect has been
> > > > > > exposed already.
> > > > > >
> > > > > > Adding a hypercall or MSR write that does a fast MMIO write to a
> > > > > > physical
> > > > > > address would do it, but it adds hypervisor knowledge in virtio,
> > > > > > including
> > > > > > CPUID handling.  So it would be pretty ugly in the guest-side
> > > > > > implementation,
> > > > > > but if somebody wants to do it and the virtio side is acceptable to the
> > > > > > virtio maintainers, I am okay with it.
> > > > > >
> > > > > > Cc: Michael S. Tsirkin <mst@xxxxxxxxxx>
> > > > > > Cc: stable@xxxxxxxxxxxxxxx
> > > > > > Fixes: 68c3b4d1676d870f0453c31d5a52e7e65c7448ae
> > > > > > Suggested-by: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
> > > > > > Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> > > > >
> > > > >
> > > > > Jason (cc) who worked on the original optimization said he can
> > > > > work to test the performance impact.
> > > > > I suggest we don't rush this (it's been like this for 2 years),
> > > > > and the issue seems to be largely theoretical.
> > > >
> > > >
> > > > Paolo, did Microsoft point it out because they hit the bug when running
> > > > KVM on Hyper-V?
> > >
> > >
> > > Does this mean the nested emulation of EPT violation and misconfiguration in
> > > KVM side doesn't strictly follow the manual since we didn't hit the bug in
> > > KVM?
> >
> > The VM-exit instruction length of vmcs12 is provided by vmcs02
> > (prepare_vmcs12()), so unless the length from vmcs02 is wrong. In
> > addition, something like mov instruction which can trigger the EPT
> > violation/misconfig in guest has already been decoded before executing
> > I think, IIUC, then exit qualification can have the information about
> > the instruction length.
>
> s/exit qualification/VM-exit instruction length

According to Paolo's comment "neither EPT violations nor 
misconfigurations are listed in the manual among the VM exits that set 
the VM-exit instruction length field", it seems to set the instruction 
length in vmcs12 is not right though it is harmless.

--
Yang
Alibaba Cloud Computing