linux-kernel - Re: [PATCH 17/24] x86/entry: Introduce struct ist

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Message-ID: <4C8A910F-679F-4F49-AAA5-AA3B9BE5F2BD@zytor.com>
Date:   Thu, 09 Sep 2021 21:51:55 -0700
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Lai Jiangshan <laijs@...ux.alibaba.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        linux-kernel@...r.kernel.org
CC:     Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, Joerg Roedel <jroedel@...e.de>,
        Youquan Song <youquan.song@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Juergen Gross <jgross@...e.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>
Subject: Re: [PATCH 17/24] x86/entry: Introduce struct ist_regs

Note: with FRED paranoid entry is no longer needed since the hardware enforces gs_base mode consistency.

On September 9, 2021 9:50:40 PM PDT, "H. Peter Anvin" <hpa@...or.com> wrote:
>It may affect your design, I don't know, but FRED exception delivery (as currently architected per draft 2.0; it is subject to change still) pushes several fields above the conventional stack frame (and thus above pt_regs.)
>
>On September 9, 2021 5:18:47 PM PDT, Lai Jiangshan <laijs@...ux.alibaba.com> wrote:
>>
>>
>>On 2021/9/1 01:50, Lai Jiangshan wrote:
>>> From: Lai Jiangshan <laijs@...ux.alibaba.com>
>>> 
>>> struct ist_regs is the upmost stack frame for IST interrupt, it
>>> consists of struct pt_regs and other fields which will be added in
>>> later patch.
>>> 
>>> Make vc_switch_off_ist() take ist_regs as its argument and it will switch
>>> the whole ist_regs if needed.
>>> 
>>> Make the frame before calling paranoid_entry() and paranoid_exit() be
>>> struct ist_regs.
>>> 
>>> This patch is prepared for converting paranoid_entry() and paranoid_exit()
>>> into C code which will need the additinal fields to store the results in
>>> paranoid_entry() and to use them in paranoid_exit().
>>
>>This patch was over designed.
>>
>>In ASM code, we can easily save results in the callee-saved registers.
>>For example, rc3 is saved in %r14, gsbase info is saved in %rbx.
>>
>>And in C code, we can't save results in registers.  And I thought there was
>>no place to save the results because the CR3 and gsbase are not kernel's.
>>So I extended the pt_regs to ist_regs to save the results.
>>
>>But it was incorrect.  The results can be saved in percpu data at the end of
>>paranoid_entry() after the CR3/gsbase are settled down.  And the results
>>can be read at the beginning of paranoid_exit() before the CR3/gsbase are
>>switched to the interrupted context's.
>>
>>sigh.
>>
>>> 
>>> The C interrupt handlers don't use struct ist_regs due to they don't need
>>> the additional fields in the struct ist_regs, and they still use pt_regs.
>>> 
>>
>

-- 
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