linux-kernel - Re: [PATCH 2/2] ARC: don't align ret_from

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Date:   Wed, 11 Mar 2020 21:10:31 +0000
From:   Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:     Eugeniy Paltsev <Eugeniy.Paltsev@...opsys.com>,
        "linux-snps-arc@...ts.infradead.org" 
        <linux-snps-arc@...ts.infradead.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Alexey Brodkin" <Alexey.Brodkin@...opsys.com>
Subject: Re: [PATCH 2/2] ARC: don't align ret_from_exception function

On 3/11/20 1:58 PM, Eugeniy Paltsev wrote:
>>
>> I would like to keep it aligned.
>>
>> ARC700 definitely has penalty for unaligned branch targets, so it will definitely
>> suffer there.
> 
> Do you know some exact numbers? I'm not an expert in ARC700 (fortunately =)

I don't remember the exact numbers either.

>> For HS, unaligned branch targets have no penalty (for the general case atleast),
>> but if it does get unaligned, the entire entry prologue code will be - i.e. each
>> one of the subsequent 130 or so instructions. That doesn't seem like a good idea
>> in general to me.
> 
> I really don't insist about applying this patch but I don't understand your
> argumentation about ARC HS like at all.

I knew you would argue hence I already copy/pasted the start and end of the
epilogue already in my prev reply which you didn't care to read thru.

If you start counting instructions from <ret_from_exception> all the way to end of
<debug_marker_ds> there are over 130 instructions all of which can be rendered
unaligned by your patch. What is worse is that this would be unpredictable: the
unaligned case will mostly NOT happen, but a new compiler or some subtle code
change could causing potentially side-effects we won't even know where to look.

>> I faked it using a nop_s and the SYM_FUNC_START_NOALIGN( ) annotation and can see
>> all instructions getting unaligned.
> 
> What is the problem with it? It's totally valid and fine for ARC HS to have instructions
> aligned by 2 byte. Or are you talking about ARC700 again?

Yes I know that already. It is fine in general as I explained earlier, but can
potentially NOT when 130 instructions are unaligned.

>> Before;
>>
>> 921238e4 <ret_from_exception>:
>> 921238e4:    ld    r8,[sp,124]
>> 921238e8:    bbit0.nt    r8,0x7,212
>> ...
>> 92123ac8:    rtie
>> 92123acc <debug_marker_ds>:
>> 92123acc:    ld    r2,[0x927d81d8]
>> 92123ad4:    add    r2,r2,0x1
>> 92123ad8:    st    r2,[0x927d81d8]
>> 92123ae0:    bmskn    r11,r10,0xf
>> 92123ae4:    sr    r11,[aux_irq_act]
>> 92123ae8:    b    -140    ;92123a5c
>>
>> After:
>>
>> 9212393c:    nop_s
>> 9212393e <ret_from_exception>:
>> 9212393e:    ld    r8,[sp,124]
>> 92123942:    bbit0.nt    r8,0x7,214
>> ...
>> 92123b22:    rtie
>> 92123b26 <debug_marker_ds>:
>> 92123b26:    ld    r2,[0x927d81d8]
>> 92123b2e:    add    r2,r2,0x1
>> 92123b32:    st    r2,[0x927d81d8]
>> 92123b3a:    bmskn    r11,r10,0xf
>> 92123b3e:    sr    r11,[aux_irq_act]
>> 92123b42:    b    -138    ;92123ab6 <debug_marker_syscall>
>> 92123b46:    nop_s