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From: "Bae, Chang Seok" <chang.seok.bae@intel.com>
To: Andy Lutomirski <luto@kernel.org>
CC: X86 ML <x86@kernel.org>, Andi Kleen <ak@linux.intel.com>,
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        "Shankar, Ravi V" <ravi.v.shankar@intel.com>,
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Subject: Re: [PATCH 14/15] x86/fsgsbase/64: Support legacy behavior when
 FS/GS updated by ptracer
Thread-Topic: [PATCH 14/15] x86/fsgsbase/64: Support legacy behavior when
 FS/GS updated by ptracer
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Date: Thu, 22 Mar 2018 15:45:25 +0000
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On 3/21/18, 18:41, "Andy Lutomirski" <luto@kernel.org> wrote:
> mov to gs changes GSBASE even if GS was unchanged.
In GDB, ptrace (syscall) doesn't happen when FS/GS unchanged as 
its (context) cache seems to be first checked. This does not allow to 
preserve GSBASE as you know.
 
> But it's not clear to me that you've identified any case where
> emulating this behavior is useful.
One argument I heard is (if debugging a legacy application) user
might want to (indirectly) access LDT during inferior call and this 
mov to fs/gs has been useful (maybe needed).