linux-kernel - Re: [PATCH] arm64: vdso32: force vdso32 to be compiled as -marm

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Message-ID: <20200528094154.GB2961@gaia>
Date:   Thu, 28 May 2020 10:41:54 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Peter Smith <Peter.Smith@....com>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Robin Murphy <Robin.Murphy@....com>,
        Will Deacon <will@...nel.org>,
        Naohiro Aota <naohiro.aota@....com>,
        Stephen Boyd <swboyd@...gle.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Manoj Gupta <manojgupta@...gle.com>,
        Luis Lozano <llozano@...gle.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Vincenzo Frascino <Vincenzo.Frascino@....com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Kristof Beyls <Kristof.Beyls@....com>,
        Victor Campos <Victor.Campos@....com>,
        "david.spickett@...aro.org" <david.spickett@...aro.org>,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH] arm64: vdso32: force vdso32 to be compiled as -marm

On Thu, May 28, 2020 at 09:05:08AM +0100, Peter Smith wrote:
> I suggest using Arm if you need a frame pointer, and disable the
> frame pointer if you want/need to use Thumb. My understanding is that
> runtime unwinding using the frame pointer in Thumb is already difficult
> due to Arm and Thumb functions using different registers for the frame
> pointer.

IIRC from the Thumb-2 kernel porting days, even in the absence of
ARM/Thumb interworking, the Thumb-2 frame pointer was pretty useless for
unwinding since it points to the bottom of the current stack frame (the
reason I think is that some LDR/STR instructions with negative indexing
are not available). So finding the previous frame pointer was not
possible and had to rely on the exception unwinding information.

-- 
Catalin