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From: Andi Kleen <ak@linux.intel.com>
Subject: Re: [PATCH 03/35] x86/entry/32: Load task stack from x86_tss.sp1 in SYSENTER handler
References: <1523892323-14741-1-git-send-email-joro@8bytes.org>
	<1523892323-14741-4-git-send-email-joro@8bytes.org>
Date: Wed, 18 Apr 2018 16:26:19 -0700
In-Reply-To: <1523892323-14741-4-git-send-email-joro@8bytes.org> (Joerg
	Roedel's message of "Mon, 16 Apr 2018 17:24:51 +0200")
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To: Joerg Roedel <joro@8bytes.org>
Cc: Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@kernel.org>, "H . Peter Anvin" <hpa@zytor.com>, x86@kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, Linus Torvalds <torvalds@linux-foundation.org>, Andy Lutomirski <luto@kernel.org>, Dave Hansen <dave.hansen@intel.com>, Josh Poimboeuf <jpoimboe@redhat.com>, Juergen Gross <jgross@suse.com>, Peter Zijlstra <peterz@infradead.org>, Borislav Petkov <bp@alien8.de>, Jiri Kosina <jkosina@suse.cz>, Boris Ostrovsky <boris.ostrovsky@oracle.com>, Brian Gerst <brgerst@gmail.com>, David Laight <David.Laight@aculab.com>, Denys Vlasenko <dvlasenk@redhat.com>, Eduardo Valentin <eduval@amazon.com>, Greg KH <gregkh@linuxfoundation.org>, Will Deacon <will.deacon@arm.com>, aliguori@amazon.com, daniel.gruss@iaik.tugraz.at, hughd@google.com, keescook@google.com, Andrea Arcangeli <aarcange@redhat.com>, Waim@linux.intel.com

Joerg Roedel <joro@8bytes.org> writes:

> From: Joerg Roedel <jroedel@suse.de>
>
> We want x86_tss.sp0 point to the entry stack later to use
> it as a trampoline stack for other kernel entry points
> besides SYSENTER.
>
> So store the task stack pointer in x86_tss.sp1, which is
> otherwise unused by the hardware, as Linux doesn't make use
> of Ring 1.

Seems like a hack. Why can't that be stored in a per cpu variable?

-Andi