linux-kernel - Re: [PATCH 7/8] x86: Add documentation for rd/wr fs/gs base

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Date:	Fri, 10 Apr 2015 13:46:07 -0700
From:	Andi Kleen <ak@...ux.intel.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Andi Kleen <andi@...stfloor.org>, X86 ML <x86@...nel.org>,
	Andrew Lutomirski <luto@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH 7/8] x86: Add documentation for rd/wr fs/gs base

> If user code programs fs != 0 and fsbase != whatever is implied by fs
> and the GDT/LDT, what happens? 

We load the value from the LDT/GDT.

> There's already a minor buglet in that
> area without wrfsbase, but fixing it would be a big performance hit
> because we don't have rdfsbase and rdgsbase to read the state
> efficiently.  (Specifically, if we have gs == 0, gsbase == 0, but
> *saved* gsbase != 0, then we corrupt gsbase on context switch.)
> 
> But, with the new instructions, we can do it simply, efficiently, and
> correctly in all cases.  Let's do so.

We would need an instruction to write the index without changing the base.
That's not what the new instructions do.

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only
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