linux-kernel - Re: [PATCH 01/16] x86/asm/64: Allocate and enable the SYSENTER stack

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Message-ID: <alpine.DEB.2.20.1711202254070.2348@nanos>
Date:   Mon, 20 Nov 2017 22:55:36 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andy Lutomirski <luto@...nel.org>
cc:     X86 ML <x86@...nel.org>, Borislav Petkov <bpetkov@...e.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Brian Gerst <brgerst@...il.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PATCH 01/16] x86/asm/64: Allocate and enable the SYSENTER
 stack

On Mon, 20 Nov 2017, Andy Lutomirski wrote:

> This will simplify future changes that want scratch variables early in
> the SYSENTER handler -- they'll be able to spill registers to the
> stack.  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.
> 
> As far as I can tell, the reason that this wasn't done from day 1 is
> that 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 want to remove IST usage from these vectors some day, and this patch
> is a prerequisite for that as well.
> 
> Signed-off-by: Andy Lutomirski <luto@...nel.org>

Reviewed-by: Thomas Gleixner <tglx@...utronix.de>