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

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Message-ID: <534827F5.1020004@intel.com>
Date:	Sat, 12 Apr 2014 01:35:49 +0800
From:	Jet Chen <jet.chen@...el.com>
To:	"H. Peter Anvin" <hpa@...or.com>,
	"Romer, Benjamin M" <Benjamin.Romer@...sys.com>
CC:	Fengguang Wu <fengguang.wu@...el.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [visorchipset] invalid opcode: 0000 [#1] PREEMPT SMP

On 04/12/2014 12:33 AM, H. Peter Anvin wrote:
> 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
> 

Hi Ben,

I re-tested this case with/without option -enable-kvm.

qemu-system-x86_64 -cpu Haswell,+smep,+smap			invalid op
qemu-system-x86_64 -cpu kvm64					invalid op
qemu-system-x86_64 -cpu Haswell,+smep,+smap -enable-kvm 	everything OK
qemu-system-x86_64 -cpu kvm64 -enable-kvm			everything OK

I think this is probably a bug in QEMU.
Sorry for misleading you. I am not experienced in QEMU usage. I don't realize I need try this case with different options Until read Peter's reply.

As Peter said, QEMU probably should *not* set the hypervisor bit. But based on my testing, I think KVM works properly in this case.

Thanks,
Jet
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