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Subject: Re: [PATCH 17/24] x86/entry: Introduce struct ist_regs
From:   Lai Jiangshan <laijs@linux.alibaba.com>
To:     Lai Jiangshan <jiangshanlai@gmail.com>,
        linux-kernel@vger.kernel.org
Cc:     Andy Lutomirski <luto@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
        Joerg Roedel <jroedel@suse.de>,
        Youquan Song <youquan.song@intel.com>,
        Tony Luck <tony.luck@intel.com>,
        Juergen Gross <jgross@suse.com>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>
References: <20210831175025.27570-1-jiangshanlai@gmail.com>
 <20210831175025.27570-18-jiangshanlai@gmail.com>
 <eb294b5d-82f2-be80-b3e3-db556c155d95@linux.alibaba.com>
Message-ID: <01b8f2d7-b987-47c5-0436-1535031fde05@linux.alibaba.com>
Date:   Fri, 10 Sep 2021 08:36:46 +0800
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On 2021/9/10 08:18, Lai Jiangshan wrote:
> 
> 
> On 2021/9/1 01:50, Lai Jiangshan wrote:
>> From: Lai Jiangshan <laijs@linux.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.

Sigh again. IST interrupts can be nested.  We can't save the results in percpu
data unless we make it stack-like which is not a good idea.  The changelog
failed to express it.

> 
>>
>> 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.
>>
>