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Subject: Re: [PATCH 11/24] x86/entry: Replace the most of asm code of
 error_entry to C code
To:     Peter Zijlstra <peterz@infradead.org>,
        Lai Jiangshan <jiangshanlai@gmail.com>
Cc:     linux-kernel@vger.kernel.org, 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>
References: <20210831175025.27570-1-jiangshanlai@gmail.com>
 <20210831175025.27570-12-jiangshanlai@gmail.com>
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From:   Lai Jiangshan <laijs@linux.alibaba.com>
Message-ID: <44ff16e6-21c8-dbd5-4393-0909162bd241@linux.alibaba.com>
Date:   Thu, 2 Sep 2021 20:08:46 +0800
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On 2021/9/2 18:16, Peter Zijlstra wrote:
> On Wed, Sep 01, 2021 at 01:50:12AM +0800, Lai Jiangshan wrote:
>> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
>> index 42d2918f5646..bc9e2f5ad370 100644
>> --- a/arch/x86/entry/entry_64.S
>> +++ b/arch/x86/entry/entry_64.S
>> @@ -972,83 +972,14 @@ SYM_CODE_START_LOCAL(error_entry)
>>   	cld
>>   	PUSH_AND_CLEAR_REGS save_ret=1
>>   	ENCODE_FRAME_POINTER 8
>>   
>>   	popq	%r12				/* save return addr in %12 */
>>   	movq	%rsp, %rdi			/* arg0 = pt_regs pointer */
>> +	call	do_error_entry
>>   	movq	%rax, %rsp			/* switch stack */
>>   	ENCODE_FRAME_POINTER
>>   	pushq	%r12
>>   	ret
> 
> There's only a single error_entry callsite, which is idtentry_body. One
> of the things I wanted to do is change this lot so we change to the
> task_stack in 'C', using an adaptation of call_on_irqstack() and
> basically don't return frrom C until we're done with \cfunc.
> 
> That is, once we call C, stay there, and don't do this back and forth
> between C and asm.

I haven't figured out how can an adaptation of call_on_irqstack() can do it.
The original stack need to be "free" for next task.  And we can't switch
the stack before error_entry() since the CR3 is not switched.

I believe the ASM code here can be simplified and clearer further. But I don't
think going back and forth between C and ASM is real issue if the ASM code is
short and simple enough.


> 
> As is, the resulting asm in error_entry is somewhat confusing given that
> we sometimes don't actually switch stacks.
>