linux-kernel - Re: Updated version of RD/WR FS/GS BASE patchkit

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Message-ID: <20160321225248.GI5083@two.firstfloor.org>
Date:	Mon, 21 Mar 2016 23:52:48 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Andi Kleen <andi@...stfloor.org>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Updated version of RD/WR FS/GS BASE patchkit

> No.  My objection is that there needs to be an explicit statement what
> the semantics are.  If the agreed-upon semantics are "undefined
> behavior if GS != 0 and GSBASE doesn't match the descriptor", so be
> it, but this needs to be a conscious decision and needs to be weighed
> against the alternatives.

Documentation/x86/fsgs.txt already has this statement:

>>>
Another requirement is that the FS or GS selector has to be zero
(is normally true unless changed explicitly). When it is non-zero
the context switch assumes the bases were loaded through the LDT/GDT,
and will reload that.
<<<

Is that sufficient?

> 
> The actual implementation details are just details.  They need to
> match the intended semantics, of course.

I believe my implementation matches the paragraph above.

-Andi

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