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From:   Vineet Gupta <Vineet.Gupta1@synopsys.com>
To:     Alexey Brodkin <Alexey.Brodkin@synopsys.com>,
        "linux-snps-arc@lists.infradead.org" 
        <linux-snps-arc@lists.infradead.org>
CC:     Alexey Brodkin <Alexey.Brodkin@synopsys.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] ARCv2: Don't pretend we may set U & DE bits in STATUS32
 with kflag
Thread-Topic: [PATCH] ARCv2: Don't pretend we may set U & DE bits in STATUS32
 with kflag
Thread-Index: AQHVPBg5vQNhnF2iB0aHFOZdtIGHIA==
Date:   Wed, 17 Jul 2019 18:35:02 +0000
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On 7/16/19 1:51 PM, Alexey Brodkin wrote:=0A=
> As per PRM "kflag" instruction doesn't change state of=0A=
> DE-flag ("Delayed branch is pending") and U-flag ("User mode")=0A=
> in STATUS32 register so let's not act as if we can affect those bits.=0A=
=0A=
I understand the motivation and indeed bits not writable by kflag should be=
=0A=
removed. But what if we do need to clear those bits out from status32 (assu=
ming=0A=
they exist there in first place) and that kflag might not be the right inst=
ruction=0A=
to do that.=0A=
So the question to ask is do we need to clear U and DE bits from status32 a=
nd=0A=
answer from reading the PRM is no, we don't have to, as those are cleared a=
lready=0A=
when an exception is taken.=0A=
=0A=
The likely reason this code is because back in arc700 days we used to have =
a=0A=
different version of this macro which atleast in original code relied on us=
ing the=0A=
pre-exception status32 (in pt_regs) - and that could easily have U and/or D=
E set=0A=
based hence needed clearing.=0A=
=0A=
.macro FAKE_RET_FROM_EXCPN  reg=0A=
=0A=
    ld  \reg, [sp, PT_status32]=0A=
    and \reg, \reg, ~(STATUS_U_MASK|STATUS_DE_MASK)=0A=
    or  \reg, \reg, STATUS_L_MASK=0A=
    sr  \reg, [erstatus]=0A=
    mov \reg, 55f=0A=
    sr  \reg, [eret]=0A=
=0A=
    rtie=0A=
55:=0A=
.endm=0A=
=0A=
This is not needed (even in arc700) if we are using the current "in-excepti=
on"=0A=
status32 for doing the early return.=0A=
=0A=
Long story short, your patch is correct but we need to explain better why i=
t is.=0A=
I've applied it locally with slight tweak to changelog to that effect.=0A=
=0A=
=0A=
> Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>=0A=
> ---=0A=
>  arch/arc/include/asm/entry-arcv2.h | 2 +-=0A=
>  1 file changed, 1 insertion(+), 1 deletion(-)=0A=
>=0A=
> diff --git a/arch/arc/include/asm/entry-arcv2.h b/arch/arc/include/asm/en=
try-arcv2.h=0A=
> index 225e7df2d8ed..6558e2edb583 100644=0A=
> --- a/arch/arc/include/asm/entry-arcv2.h=0A=
> +++ b/arch/arc/include/asm/entry-arcv2.h=0A=
> @@ -237,7 +237,7 @@=0A=
>  =0A=
>  .macro FAKE_RET_FROM_EXCPN=0A=
>  	lr      r9, [status32]=0A=
> -	bic     r9, r9, (STATUS_U_MASK|STATUS_DE_MASK|STATUS_AE_MASK)=0A=
> +	bic     r9, r9, STATUS_AE_MASK=0A=
>  	or      r9, r9, STATUS_IE_MASK=0A=
>  	kflag   r9=0A=
>  .endm=0A=
=0A=