linux-kernel - Re: [PATCH v1] ARM: vfp: Use long jump to fix THUMB2 kernel compilation error

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Message-ID: <f4b4f5ca-2c0c-c1f0-9329-a95967b99bd0@gmail.com>
Date:   Tue, 3 Nov 2020 11:43:10 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Ard Biesheuvel <ardb@...nel.org>,
        Russell King - ARM Linux admin <linux@...linux.org.uk>
Cc:     Kees Cook <keescook@...omium.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Ingo Molnar <mingo@...nel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v1] ARM: vfp: Use long jump to fix THUMB2 kernel
 compilation error

03.11.2020 10:24, Ard Biesheuvel пишет:
> Still broken today
> 
> https://kernelci.org/build/id/5fa0c1a74bdb1ea4063fe7e4/
> 
> So the options are
> 
> a) merge my patch that adds 2 bytes of opcode to the Thumb2 build
> b) merge Dmitry's patch that adds an unconditional literal load to all builds
> c) remove kernel mode handling from vfp_support_entry() [my other patch]
> d) move sections around so that vfp_kmode_exception is guaranteed to
> be in range.
> e) do nothing
> 
> Given the lack of reports about this issue, it is pretty clear that
> few people use the Thumb2 build (which I find odd, tbh, since it
> really is much smaller).

I waited for about a month, hoping that somebody will fix this problem
before bothering with bisection, which took quite some time and effort
because intermediate commits were broken, and then with creating and
sending a patch :)

Thumb2 usually is untested by CI farms and in a case of personal use
it's easier to wait for a fix. Hence no much reports, I suppose.

> However, that means that a) is a reasonable
> fix, since nobody will notice the potential performance hit either,
> and it can easily be backported to wherever the breakage was
> introduced. (Note that eff8728fe698, which created the problem is
> marked cc:stable itself).

The performance argument is questionable to me, to be honest. In
practice the performance difference should be absolutely negligible for
either of the proposed options, it should stay in a noise even if
somebody thoroughly counting cycles, IMO.

I'm still thinking that the best option will be to apply a).

> Going forward, I can refine d) so that we can get rid of the kernel
> mode path entirely.

And then improve it using d).