Re: [visorchipset] invalid opcode: 0000 [#1] PREEMPT SMP

[H. Peter Anvin]

On 04/11/2014 06:51 AM, Romer, Benjamin M wrote:
>
>> I'm still confused where KVM comes into the picture.  Are you actually
>> using KVM (and thus talking about nested virtualization) or are you
>> using Qemu in JIT mode and running another hypervisor underneath?
> 
> The test that Fengguang used to find the problem was running the linux
> kernel directly using KVM. When the kernel was run with "-cpu Haswell,
> +smep,+smap" set, the vmcall failed with invalid op, but when the kernel
> is run with "-cpu qemu64", the vmcall causes a vmexit, as it should.

As far as I know, Fengguang's test doesn't use KVM at all, it runs Qemu
as a JIT.  Completely different thing.  In that case Qemu probably
should *not* set the hypervisor bit.  However, the only thing that the
hypervisor bit means is that you can look for specific hypervisor APIs
in CPUID level 0x40000000+.

> My point is, the vmcall was made because the hypervisor bit was set. If
> this bit had been turned off, as it would be on a real processor, the
> vmcall wouldn't have happened.

And my point is that that is a bug.  In the driver.  A very serious one.
 You cannot call VMCALL until you know *which* hypervisor API(s) you
have available, period.

>> The hypervisor bit is a complete red herring. If the guest CPU is
>> running in VT-x mode, then VMCALL should VMEXIT inside the guest
>> (invoking the guest root VT-x), 
> 
> The CPU is running in VT-X. That was my point, the kernel is running in
> the KVM guest, and KVM is setting the CPU feature bits such that bit 31
> is enabled.

Which it is because it wants to export the KVM hypercall interface.
However, keying VMCALL *only* on the HYPERVISOR bit is wrong in the extreme.

> I don't think it's a red herring because the kernel uses this bit
> elsewhere - it is reported as X86_FEATURE_HYPERVISOR in the CPU
> features, and can be checked with the cpu_has_hypervisor macro (which
> was not used by the original author of the code in the driver, but
> should have been). VMWare and KVM support in the kernel also check for
> this bit before checking their hypervisor leaves for an ID. If it's not
> properly set it affects more than just the s-Par drivers.
> 
>> but the fact still remains that you
>> should never, ever, invoke VMCALL unless you know what hypervisor you
>> have underneath.
> 
> From the standpoint of the s-Par drivers, yes, I agree (as I already
> said). However, VMCALL is not a privileged instruction, so anyone could
> use it from user space and go right past the OS straight to the
> hypervisor. IMHO, making it *lethal* to the guest is a bad idea, since
> any user could hard-stop the guest with a couple of lines of C.

Typically the hypervisor wants to generate a #UD inside of the guest for
that case.  The guest OS will intercept it and SIGILL the user space
process.

	-hpa

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