LKML Archive on lore.kernel.org
help / color / mirror / Atom feed
From: Vineet Gupta <Vineet.Gupta1@synopsys.com>
To: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>,
	"linux-snps-arc@lists.infradead.org" 
	<linux-snps-arc@lists.infradead.org>
Cc: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"Alexey Brodkin" <Alexey.Brodkin@synopsys.com>
Subject: Re: [PATCH 2/2] ARC: don't align ret_from_exception function
Date: Wed, 11 Mar 2020 21:10:31 +0000	[thread overview]
Message-ID: <a6728afe-10e4-ce85-b133-17395ac99b86@synopsys.com> (raw)
In-Reply-To: <BY5PR12MB40348292DA303489C1DA2B66DEFC0@BY5PR12MB4034.namprd12.prod.outlook.com>

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

  reply	other threads:[~2020-03-11 21:10 UTC|newest]

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-11 16:26 [PATCH 1/2] ARC: define __ALIGN_STR and __ALIGN symbols for ARC Eugeniy Paltsev
2020-03-11 16:26 ` [PATCH 2/2] ARC: don't align ret_from_exception function Eugeniy Paltsev
2020-03-11 17:38   ` Vineet Gupta
2020-03-11 20:58     ` Eugeniy Paltsev
2020-03-11 21:10       ` Vineet Gupta [this message]
2020-03-11 22:28         ` Eugeniy Paltsev
2020-03-11 16:35 ` [PATCH 1/2] ARC: define __ALIGN_STR and __ALIGN symbols for ARC Vineet Gupta

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=a6728afe-10e4-ce85-b133-17395ac99b86@synopsys.com \
    --to=vineet.gupta1@synopsys.com \
    --cc=Alexey.Brodkin@synopsys.com \
    --cc=Eugeniy.Paltsev@synopsys.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-snps-arc@lists.infradead.org \
    --subject='Re: [PATCH 2/2] ARC: don'\''t align ret_from_exception function' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).