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Date: Wed, 18 Apr 2018 17:38:33 -0700
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
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To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Joerg Roedel <joro@8bytes.org>, Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@kernel.org>, "H . Peter Anvin" <hpa@zytor.com>, the arch/x86 maintainers <x86@kernel.org>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, linux-mm <linux-mm@kvack.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>, "Liguori, Anthony" <aliguori@amazon.com>, Daniel Gruss <daniel.gruss@iaik.tugraz.at>, Hugh Dickins <hughd@google.com>, Kees Cook <keescook@google.com>, Andrea Arcangeli <aarcange@redhat.com>, Waim@linux.intel.com

On Wed, Apr 18, 2018 at 05:02:02PM -0700, Linus Torvalds wrote:
> On Wed, Apr 18, 2018 at 4:26 PM, Andi Kleen <ak@linux.intel.com> wrote:
> >
> > Seems like a hack. Why can't that be stored in a per cpu variable?
> 
> It *is* a percpu variable - the whole x86_tss structure is percpu.
> 
> I guess it could be a different (separate) percpu variable, but might
> as well use the space we already have allocated.

Would be better/cleaner to use a separate variable instead of reusing
x86 structures like this. Who knows what subtle side effects that
may have eventually.

It will be also easier to understand in the code.

-Andi