linux-kernel - Re: [x86] 5ac0c41bf3: WARNING: CPU: 0 PID: 0 at arch/x86/mm/extable.c:50 ex_handler_rdmsr

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Date:	Wed, 15 Jun 2016 16:53:01 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Borislav Petkov <bp@...en8.de>,
	kernel test robot <fengguang.wu@...el.com>,
	Andy Lutomirski <luto@...capital.net>,
	Eduardo Habkost <ehabkost@...hat.com>
Cc:	LKP <lkp@...org>, wfg@...ux.intel.com,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [x86] 5ac0c41bf3: WARNING: CPU: 0 PID: 0 at
 arch/x86/mm/extable.c:50 ex_handler_rdmsr_unsafe



On 15/06/2016 16:25, Borislav Petkov wrote:
> As to the error message, dear LKP friends, it happens because -cpu kvm64
> on native Intel hands in CPUID bits of the host, i.e., if you do this in
> the guest:
> 
> $ grep epb /proc/cpuinfo
> flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 syscall nx lm constant_tsc nopl eagerfpu pni cx16 x2apic hypervisor epb
> 
> you should have "epb" there too which is among those bits.

Hmm, no, it doesn't work like that.

EPB is bit 3 of CPUID[6].ECX.  Under KVM you should only ever see bit 2
of EAX set in that leaf (ARAT).

> I can reproduce the same issue on an AMD host too by booting my guest
> with
> 
> "-cpu kvm64,vendor=GenuineIntel"

I cannot reproduce it with 4.6.0-rc3 in the (Fedora 22 AMD) host and
4.7.0-rc2 in the (Fedora 21) guest.  QEMU is 2.4.1.

> Paolo, Eduardo, question: can we hide certain CPUID bits from the guest
> when booting with -cpu kvm64?
> 
> In general, is there a way I can set or clear arbitrary CPUID bits so
> that the guest sees what I want it to see?
> 
> And I don't mean the predefined CPUID flags which you toggle with "+" or
> "-" followed by flag name. Because -cpu kvm64,-epb doesn't work.

It doesn't work because QEMU has no idea of what EPB even is.  That bit
really shouldn't be set.

Can you bounce me the original report?

Paolo