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Date: Fri, 24 Nov 2017 14:47:40 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Ingo Molnar <mingo@kernel.org>
Cc: linux-kernel@vger.kernel.org,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Andy Lutomirski <luto@amacapital.net>,
        Thomas Gleixner <tglx@linutronix.de>,
        "H . Peter Anvin" <hpa@zytor.com>, Borislav Petkov <bp@alien8.de>,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH 30/43] x86/mm/kaiser: Map espfix structures
Message-ID: <20171124134740.6dmv6nfo5ihrkj3f@hirez.programming.kicks-ass.net>
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 <20171124091448.7649-31-mingo@kernel.org>
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On Fri, Nov 24, 2017 at 10:14:35AM +0100, Ingo Molnar wrote:
> From: Dave Hansen <dave.hansen@linux.intel.com>
> 
> There is some rather arcane code to help when an IRET returns
> to 16-bit segments.  It is referred to as the "espfix" code.
> This consists of a few per-cpu variables:
> 
> 	espfix_stack: tells us where the stack is allocated
> 		      (the bottom)
> 	espfix_waddr: tells us to where %rsp may be pointed
> 		      (the top)
> 
> These are in addition to the stack itself.  All three things must
> be mapped for the espfix code to function.
> 
> Note: the espfix code runs with a kernel GSBASE, but user
> (shadow) page tables.  A switch to the kernel page tables could
> be performed instead of mapping these structures, but mapping
> them is simpler and less likely to break the assembly.  To switch
> over to the kernel copy, additional temporary storage would be
> required which is in short supply in this context.

With Andy's patches that should actually be doable, no?