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

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Date:	Mon, 21 Mar 2016 15:57:40 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	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

On Mon, Mar 21, 2016 at 3:52 PM, Andi Kleen <andi@...stfloor.org> wrote:
>> 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?
>

Maybe.  Are there better options?  Could we, for example, actually try
to preserve the state if this happens?  Would it be worth it?

>>
>> 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.



-- 
Andy Lutomirski
AMA Capital Management, LLC