linux-kernel - Re: [PATCH] x86/TSC: Use RDTSCP

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Date:   Fri, 23 Nov 2018 21:44:25 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Guenter Roeck <linux@...ck-us.net>
Cc:     X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        John Stultz <john.stultz@...aro.org>,
        Thomas Lendacky <Thomas.Lendacky@....com>
Subject: Re: [PATCH] x86/TSC: Use RDTSCP

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?

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.

-- 
Regards/Gruss,
    Boris.

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