linux-kernel - Re: [x86] fs, gs purpose & multicore prog

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Date:	Sat, 6 Sep 2008 01:09:52 +0200
From:	Eric Lacombe <goretux@...il.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [x86] fs, gs purpose & multicore prog

Thanks again ;)

On Vendredi 05 Septembre 2008 16:51:39 Jeremy Fitzhardinge wrote:
> Eric Lacombe wrote:
[...]
> >>> - What is the purpose of MSR_FS_BASE and MSR_GS_BASE ?
> >>> (I thought they were filled with "gdt[fs_entry].base")
> >>
> >> On 64-bit, the GDT isn't large enough to hold a 64-bit offset, so it
> >> only stores the low 32-bits.  When you load a segment register with a
> >> selector, it picks up from the gdt.  If you want a full 64-bit offset,
> >> you need to write it to the msr.
> >
> > Ok, I just saw that a 64-bit base in segment descriptor is only available
> > for the system descriptor.
>
> Yes, the IDT has double-wide entries to fit 64-bit values, but they
> didn't extend that to the GDT.  Or something - I last looked at this a
> couple of months ago, and it never sticks in my brain for long.

It seems, there also are the TSS and LDT.

>
> > Ok, but how does the kernel technically run tasks on different processor
> > (or core)? My question was ambiguous, I was not assuming that I knew how
> > multiprocessor works.
>
> That's a very broad question.  A good proportion of the core kernel code
> is dedicated to doing just that.  Very roughly, at boot time it brings
> up all the cpus, and they more or less run independently each looking
> for work to do in the form of processes waiting to run on the run
> queue.  They collectively run the scheduler algorithms to work out who
> runs what when; almost everything run in the kernel is a task - both
> usermode processes and kernel threads.  Except for the stuff which isn't.

I know these things ;) but what I wanted to know is the "x86 architectural 
details". In fact, I saw that during the machine init the BIOS select a cpu on 
the bus to be the BSP (bootstrap proc). The others are then the APs (Appli 
proc). Then the kernel runs on the BSP. What I wonder is how the kernel gives 
execution flow to the APs.

Thanks.

	Eric

>
>     J

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