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

[Thomas Gleixner]

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@xxxxxxxxxx>

Reviewed-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>