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From: Andy Lutomirski <luto@kernel.org>
Date: Mon, 13 Nov 2017 18:17:44 -0800
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Subject: Re: [RFC 1/7] x86/asm/64: Allocate and enable the SYSENTER stack
To: Dave Hansen <dave.hansen@intel.com>
Cc: Andy Lutomirski <luto@kernel.org>, X86 ML <x86@kernel.org>,
        Borislav Petkov <bpetkov@suse.de>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Brian Gerst <brgerst@gmail.com>,
        Linus Torvalds <torvalds@linux-foundation.org>
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On Mon, Nov 13, 2017 at 11:07 AM, Dave Hansen <dave.hansen@intel.com> wrote:
> On 11/10/2017 08:05 PM, Andy Lutomirski wrote:
>> This will simplify some future code changes that will want some
>> temporary stack space in more places.  It also lets us get rid of a
>> SWAPGS_UNSAFE_STACK user.
>>
>> This does not depend on CONFIG_IA32_EMULATION because we'll want the
>> stack space even without IA32 emulation.
>
> It was never clear to me why we don't use this on 64-bit today.  Does
> anybody know why?

Nothing used it?

As far as I can tell, the original x86_64 Linux port was a little bit
more excited about IST than I think made sense.  As a result, we use
IST for #DB and #BP, which is IMO rather nasty and causes a lot more
problems than it solves.  But, since #DB uses IST, we don't actually
need a real stack for SYSENTER (because SYSENTER with TF set will
invoke #DB on the IST stack rather than the SYSENTER stack).

I have old patches to stop using IST for #DB and #BP, but I never finished them.