linux-kernel - Re: [PATCH 30/43] x86/mm/kaiser: Map espfix structures

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Date:   Fri, 24 Nov 2017 08:17:06 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H . Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...en8.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 30/43] x86/mm/kaiser: Map espfix structures

On Fri, Nov 24, 2017 at 5:47 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Fri, Nov 24, 2017 at 10:14:35AM +0100, Ingo Molnar wrote:
>> From: Dave Hansen <dave.hansen@...ux.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?

I don't think it has much to do with my patches.  We can freely spill
to the stack in the espfix64 code, though.

--Andy