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Message-ID: <20120817080956.GC24241@liondog.tnic>
Date: Fri, 17 Aug 2012 10:09:56 +0200
From: Borislav Petkov <bp@...en8.de>
To: Tomas Racek <tracek@...hat.com>
Cc: Anthony Liguori <anthony@...emonkey.ws>, qemu-devel@...gnu.org,
"H. Peter Anvin" <hpa@...ux.intel.com>,
linux-kernel@...r.kernel.org, Alan Cox <alan@...ux.intel.com>,
kvm-devel <kvm@...r.kernel.org>, Avi Kivity <avi@...hat.com>,
Marcelo Tosatti <mtosatti@...hat.com>,
Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [Qemu-devel] x86, nops settings result in kernel crash
On Fri, Aug 17, 2012 at 03:43:56AM -0400, Tomas Racek wrote:
> Well, I've added some debug statements to the code:
>
> void __init arch_init_ideal_nops(void)
> {
> switch (boot_cpu_data.x86_vendor) {
> case X86_VENDOR_INTEL:
> /*
> * Due to a decoder implementation quirk, some
> * specific Intel CPUs actually perform better with
> * the "k8_nops" than with the SDM-recommended NOPs.
> */
> if (boot_cpu_data.x86 == 6 &&
> boot_cpu_data.x86_model >= 0x0f &&
> boot_cpu_data.x86_model != 0x1c &&
> boot_cpu_data.x86_model != 0x26 &&
> boot_cpu_data.x86_model != 0x27 &&
> boot_cpu_data.x86_model < 0x30) {
> printk("NOPS: Option 1\n");
> ideal_nops = k8_nops;
> } else if (boot_cpu_has(X86_FEATURE_NOPL)) {
> printk("NOPS: Option 2\n");
> ideal_nops = p6_nops;
> } else {
> printk("NOPS: Option 3\n");
> #ifdef CONFIG_X86_64
> ideal_nops = k8_nops;
> #else
> ideal_nops = intel_nops;
> #endif
> }
> break;
> default:
> #ifdef CONFIG_X86_64
> ideal_nops = k8_nops;
> #else
> if (boot_cpu_has(X86_FEATURE_K8))
> ideal_nops = k8_nops;
> else if (boot_cpu_has(X86_FEATURE_K7))
> ideal_nops = k7_nops;
> else
> ideal_nops = intel_nops;
> #endif
> }
> }
>
> This gives me Option 1 with "-cpu host" and Option 2 without.
This looks like an emulation bug. The interesting thing is that your
both traces from the bugzilla point to generic_make_request_checks but
it could also be due to timing.
Decoding the instruction stream in the second trace in the bugzilla gives:
[ 278.595106] Code: 03 48 89 03 48 8b 57 70 48 89 53 10 48 2b 01 8b 3f 48 89 45 98 48 8b 82 90 00 00 00 89 7d 94 48 8b 80 60 02 00 00 48 89 45 88 ac <17> 00 00 c8 45 85 e4 74 30 48 8b 43 10 48 8b 40 08 48 8b 40 48
All code
========
0: 03 48 89 add -0x77(%rax),%ecx
3: 03 48 8b add -0x75(%rax),%ecx
6: 57 push %rdi
7: 70 48 jo 0x51
9: 89 53 10 mov %edx,0x10(%rbx)
c: 48 2b 01 sub (%rcx),%rax
f: 8b 3f mov (%rdi),%edi
11: 48 89 45 98 mov %rax,-0x68(%rbp)
15: 48 8b 82 90 00 00 00 mov 0x90(%rdx),%rax
1c: 89 7d 94 mov %edi,-0x6c(%rbp)
1f: 48 8b 80 60 02 00 00 mov 0x260(%rax),%rax
26: 48 89 45 88 mov %rax,-0x78(%rbp)
2a: ac lods %ds:(%rsi),%al
2b:* 17 (bad) <-- trapping instruction
2c: 00 00 add %al,(%rax)
2e: c8 45 85 e4 enterq $0x8545,$0xe4
32: 74 30 je 0x64
34: 48 8b 43 10 mov 0x10(%rbx),%rax
38: 48 8b 40 08 mov 0x8(%rax),%rax
3c: 48 8b 40 48 mov 0x48(%rax),%rax
...
Code starting with the faulting instruction
===========================================
0: 17 (bad)
1: 00 00 add %al,(%rax)
3: c8 45 85 e4 enterq $0x8545,$0xe4
7: 74 30 je 0x39
9: 48 8b 43 10 mov 0x10(%rbx),%rax
d: 48 8b 40 08 mov 0x8(%rax),%rax
11: 48 8b 40 48 mov 0x48(%rax),%rax
and an instruction with opcode 0x17 in 64-bit mode is, AFAICT,
invalid (on 32-bit it is "pop %ss" according to this thing:
http://www.onlinedisassembler.com).
--
Regards/Gruss,
Boris.
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