LE/GE (exact breakpoint detection)

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ouadji

Joined: 24 Dec 2008
Posts: 1080
Location: Belgium

ouadji 23 May 2011, 16:43

Quote:

3A/16.2.4
-----------
LE and GE (local and global exact breakpoint enable) flags (bits 8, 9) —
This feature is not supported in the P6 family processors, later IA-32 processors, and Intel 64 processors. When set, these flags cause the processor to detect the exact instruction that caused a data breakpoint condition. For backward and forward compatibility with other Intel processors, we recommend that the LE and GE flags be set to 1 if exact breakpoints are required.

Does anyone know exactly how this feature works ?
What are the differences compared to a normal detection ?

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I am not young enough to know everything (Oscar Wilde)-

23 May 2011, 16:43

Feryno

Joined: 23 Mar 2005
Posts: 518
Location: Czech republic, Slovak republic

Feryno 24 May 2011, 05:32

they are not used by hardware in newer CPUs anymore...
but somebody clever from microsoft kernel developpers team used them for another great debugging feature - shadows of DebugCtl.LBR, DebugCtl.LBR
DebugCtl control register in ms windows belong to CPU/core, debug registers belong to thread, so there is a way to save LBR, BTF into thread context /restore from thread context for every thread no matter rescheduling thread at any CPU/core
http://x86asm.net/articles/backdoor-support-for-control-transfer-breakpoint-features/index.html

24 May 2011, 05:32

ouadji

Joined: 24 Dec 2008
Posts: 1080
Location: Belgium

ouadji 24 May 2011, 06:47

a very interesting web page, really !
thank you Feryno for your reply and this link.

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I am not young enough to know everything (Oscar Wilde)-

24 May 2011, 06:47

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