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SubjectRe: [PATCH] x86/TSC: Use RDTSCP
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On 11/23/18 12:44 PM, Borislav Petkov wrote:
> On Fri, Nov 23, 2018 at 12:03:07PM -0800, Guenter Roeck wrote:
>> [ 0.762832] EIP: read_tsc+0x4/0x10
>> [ 0.762832] Code: 00 01 00 eb 89 90 55 89 e5 5d c3 90 90 90 90 90 90 90 90 90 90 90 55 a1 44 5a 8b c5 89 e5 5d c3 8d b6 00 00 00 00 55 89 e5 57 <0f> ae f0b
>
> Where does that 'b' in f0b come from?
>

Good catch.

It is a cut off screen log. x86 boots change xterm configuration from
wrap to non-wrap, and I did a cut-and-paste instead of copying the log
to a file. Sorry for that.

Guenter

> But ok, I was able to reproduce and decode myself. So if the Code:
> section is correct, qemu chokes on MFENCE.
>
> [ 0.854209] Code: 90 90 90 90 90 90 90 a1 84 37 11 cd c3 8d b4 26 00 00 00 00 8d 76 00 c3 8d b4 26 00 00 00 00 8d b4 26 00 00 00 00 90 55 89 e5 <0f> ae f0 0f 31 5d c3 8d b6 00 00 00 00 55 89 e5 57 31 ff 56 53 89
> All code
> ========
> 0: 90 nop
> 1: 90 nop
> 2: 90 nop
> 3: 90 nop
> 4: 90 nop
> 5: 90 nop
> 6: 90 nop
> 7: a1 84 37 11 cd mov 0xcd113784,%eax
> c: c3 ret
> d: 8d b4 26 00 00 00 00 lea 0x0(%esi,%eiz,1),%esi
> 14: 8d 76 00 lea 0x0(%esi),%esi
> 17: c3 ret
> 18: 8d b4 26 00 00 00 00 lea 0x0(%esi,%eiz,1),%esi
> 1f: 8d b4 26 00 00 00 00 lea 0x0(%esi,%eiz,1),%esi
> 26: 90 nop
> 27: 55 push %ebp
> 28: 89 e5 mov %esp,%ebp
> 2a:* 0f ae f0 mfence <-- trapping instruction
> 2d: 0f 31 rdtsc
> 2f: 5d pop %ebp
> 30: c3 ret
> 31: 8d b6 00 00 00 00 lea 0x0(%esi),%esi
> 37: 55 push %ebp
> 38: 89 e5 mov %esp,%ebp
> 3a: 57 push %edi
> 3b: 31 ff xor %edi,%edi
> 3d: 56 push %esi
> 3e: 53 push %ebx
> 3f: 89 .byte 0x89
>
> Doing this:
>
> asm volatile(ALTERNATIVE_2("mfence", ...
>
> fails too which confirms that P3 can't do MFENCE.
>
> I need to think about how to handle that old cruft.
>
> Thx for the report.
>

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