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

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Date:	Wed, 30 Apr 2014 12:02:34 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	"H. Peter Anvin" <hpa@...or.com>, Jet Chen <jet.chen@...el.com>,
	"Romer, Benjamin M" <Benjamin.Romer@...sys.com>
CC:	Fengguang Wu <fengguang.wu@...el.com>,
	Borislav Petkov <bp@...en8.de>,
	LKML <linux-kernel@...r.kernel.org>,
	qemu-devel <qemu-devel@...gnu.org>
Subject: Re: [visorchipset] invalid opcode: 0000 [#1] PREEMPT SMP

Il 11/04/2014 19:40, H. Peter Anvin ha scritto:
> On 04/11/2014 10:35 AM, Jet Chen wrote:
>>
>> As Peter said, QEMU probably should *not* set the hypervisor bit. But based on my testing, I think KVM works properly in this case.
>>
>
> Either way, unless there is a CPUID interface exposed in CPUID levels
> 0x40000000+, then relying on the hypervisor bit to do VMCALL is wrong in
> the extreme.

Sorry for the delay guys, I was on vacation.

Lack of a CPUID interface at 0x40000000 is indeed *the* good reason why 
QEMU should not set the hypervisor bit.  Of course that there is no 
guarantee that QEMU will never expose a 0x40000000 interface, and at 
that point the hypervisor bit may reappear in QEMU's JIT mode.

As to sending #UD to the guest at CPL>0, that is a choice of the 
hypervisor.  Hyper-V (and KVM in Hyper-V emulation mode) does that, and 
does the same in real mode too.  KVM instead sets EAX to -KVM_EPERM, and 
accepts hypercalls in real mode (where CPL=0).  Terminating the guest is 
surely the wrong thing to do at CPL>0.

Thanks,

Paolo
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