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

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Message-ID: <20160615142532.GE32588@pd.tnic>
Date:	Wed, 15 Jun 2016 16:25:32 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	kernel test robot <fengguang.wu@...el.com>,
	Andy Lutomirski <luto@...capital.net>,
	Paolo Bonzini <pbonzini@...hat.com>,
	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 Wed, Jun 15, 2016 at 08:25:57PM +0800, kernel test robot wrote:
> [    0.556833] Mountpoint-cache hash table entries: 1024 (order: 0, 4096 bytes)
> [    0.559888] ------------[ cut here ]------------
> [    0.559888] ------------[ cut here ]------------
> [    0.561405] WARNING: CPU: 0 PID: 0 at arch/x86/mm/extable.c:50 ex_handler_rdmsr_unsafe+0x44/0x70
> [    0.561405] WARNING: CPU: 0 PID: 0 at arch/x86/mm/extable.c:50 ex_handler_rdmsr_unsafe+0x44/0x70
> [    0.567649] unchecked MSR access error: RDMSR from 0x1b0
> [    0.567649] unchecked MSR access error: RDMSR from 0x1b0

Btw, Andy, this error message is completely useless - I
wanna know *where* the RDMSR in the code is, not point me at
ex_handler_rdmsr_unsafe().

IOW, I wanna convert the current thing into this:

[    0.028003] unchecked MSR access error: RDMSR from 0x1b0 at rIP: 0xffffffff81026d9f
[    0.030343] ENERGY_PERF_BIAS: Set to 'normal', was 'performance'
[    0.032003] ENERGY_PERF_BIAS: View and update with x86_energy_perf_policy(8)
[    0.036003] unchecked MSR access error: WRMSR to 0x1b0 (tried to write 0x0000000000000006) at rIP: 0xffffffff81026de1

i.e.,

---
diff --git a/arch/x86/mm/extable.c b/arch/x86/mm/extable.c
index 4bb53b89f3c5..2028a5ad3433 100644
--- a/arch/x86/mm/extable.c
+++ b/arch/x86/mm/extable.c
@@ -46,8 +46,8 @@ EXPORT_SYMBOL(ex_handler_ext);
 bool ex_handler_rdmsr_unsafe(const struct exception_table_entry *fixup,
 			     struct pt_regs *regs, int trapnr)
 {
-	WARN_ONCE(1, "unchecked MSR access error: RDMSR from 0x%x\n",
-		  (unsigned int)regs->cx);
+	pr_warn_once("unchecked MSR access error: RDMSR from 0x%x at rIP: 0x%lx\n",
+		     (unsigned int)regs->cx, regs->ip);
 
 	/* Pretend that the read succeeded and returned 0. */
 	regs->ip = ex_fixup_addr(fixup);
@@ -60,9 +60,9 @@ EXPORT_SYMBOL(ex_handler_rdmsr_unsafe);
 bool ex_handler_wrmsr_unsafe(const struct exception_table_entry *fixup,
 			     struct pt_regs *regs, int trapnr)
 {
-	WARN_ONCE(1, "unchecked MSR access error: WRMSR to 0x%x (tried to write 0x%08x%08x)\n",
-		  (unsigned int)regs->cx,
-		  (unsigned int)regs->dx, (unsigned int)regs->ax);
+	pr_warn_once("unchecked MSR access error: WRMSR to 0x%x (tried to write 0x%08x%08x) at rIP: 0x%lx\n",
+		     (unsigned int)regs->cx, (unsigned int)regs->dx,
+		     (unsigned int)regs->ax,  regs->ip);
 
 	/* Pretend that the write succeeded. */
 	regs->ip = ex_fixup_addr(fixup);
---

Ok?

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.

I can reproduce the same issue on an AMD host too by booting my guest
with

"-cpu kvm64,vendor=GenuineIntel"

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.

Is there a way to make this work or should we hack it into qemu so that
we are able to do that? I.e., something like

-cpu=kvm64,cpuid=leaf6_ecx_bit3=0,...

or something smarter. But you get the idea...

Thanks.

-- 
Regards/Gruss,
    Boris.

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