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From: Eric Lacombe <goretux@gmail.com>
To: Jeremy Fitzhardinge <jeremy@goop.org>,
       Arjan van de Ven <arjan@infradead.org>, Ingo Molnar <mingo@elte.hu>,
       Alan Cox <alan@lxorguk.ukuu.org.uk>
Subject: Re: [x86] do_arch_prctl
Date: Mon, 24 Nov 2008 13:24:53 +0100
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Cc: linux-kernel@vger.kernel.org
References: <200811181835.07360.goretux@gmail.com> <4924AA4E.4090001@goop.org> <200811200122.07694.goretux@gmail.com>
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Hello,

Does the "doit case" (line 822 in ARCH_GET_FS, function do_arch_prctl) exist 
for performance reasons? Else, why "task->thread.fs" (line 824) does not 
contain the fs base in the "doit case"?

Can someone explain _precisely_ the lines 835 through 838 (ARCH_GET_GS)?
(I thought that just the line 836 was sufficient, but I 
obviously miss the case where MSR_KERNEL_GS_BASE does not reflect the value 
requested)

Thanks again for all your previous answers.

	Eric

828 case ARCH_GET_GS: {
829                 unsigned long base;
830                 unsigned gsindex;
831                 if (task->thread.gsindex == GS_TLS_SEL)
832                         base = read_32bit_tls(task, GS_TLS);
833                 else if (doit) {
834                         asm("movl %%gs,%0" : "=r" (gsindex));
835                         if (gsindex)
836                                 rdmsrl(MSR_KERNEL_GS_BASE, base);
837                         else
838                                 base = task->thread.gs;
839                 }
840                 else
841                         base = task->thread.gs;
842                 ret = put_user(base, (unsigned long __user *)addr);
843                 break;
844         }

---

817         case ARCH_GET_FS: {
818                 unsigned long base;
819                 if (task->thread.fsindex == FS_TLS_SEL)
820                         base = read_32bit_tls(task, FS_TLS);
821                 else if (doit)
822                         rdmsrl(MSR_FS_BASE, base);
823                 else
824                         base = task->thread.fs;
825                 ret = put_user(base, (unsigned long __user *)addr);
826                 break;
827         }

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