Date:   Fri, 4 Nov 2016 08:26:03 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Matthew Fortune <Matthew.Fortune@...tec.com>
Cc:     Maciej Rozycki <Maciej.Rozycki@...tec.com>,
        Ralf Baechle <ralf@...ux-mips.org>,
        Alex Smith <Alex.Smith@...tec.com>,
        "linux-mips@...ux-mips.org" <linux-mips@...ux-mips.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        James Hogan <James.Hogan@...tec.com>
Subject: Re: [PATCH] MIPS: VDSO: Always select -msoft-float

On Fri, Nov 04, 2016 at 01:42:40PM +0000, Matthew Fortune wrote:
> Maciej Rozycki <Maciej.Rozycki@...tec.com> writes:
> > On Tue, 1 Nov 2016, Guenter Roeck wrote:
> > 
> > > > > Some toolchains fail to build mips images with the following build
> > error.
> > > > >
> > > > > arch/mips/vdso/gettimeofday.c:1:0: error: '-march=r3000' requires
> > '-mfp32'
> > > > >
> > > > > This is seen, for example, with the 'mipsel-linux-gnu-gcc (Debian
> > > > > 6.1.1-9)
> > > > > 6.1.1 20160705' toolchain as used by the 0Day build robot when
> > > > > building decstation_defconfig.
> > > > >
> > > > > Comparison of compile flags suggests that the major difference is
> > > > > a missing '-soft-float', which is otherwise defined
> > unconditionally.
> > > > >
> > > > > Reported-by: kbuild test robot <fengguang.wu@...el.com>
> > > > > Cc: James Hogan <james.hogan@...tec.com>
> > > > > Fixes: ebb5e78cc634 ("MIPS: Initial implementation of a VDSO")
> > > > > Signed-off-by: Guenter Roeck <linux@...ck-us.net>
> > > > > ---
> > [...]
> > > >  If you send me the generated assembly, i.e. `gettimeofday.s', that
> > > > is causing you trouble, then I'll see if I can figure out what is
> > > > going on here.  We may decide to paper a particularly nasty
> > > > toolchain bug over from
> > >
> > > The problem is seen in 0Day builds, with the toolchain mentioned
> > above.
> > 
> >  That does not tell me anything -- I have no idea how each of the
> > toolchains out there has been configured.  I can only assess information
> > provided by the bug/patch submitter, and otherwise try guessing.
> > 
> > > I don't think that generated assembly is going to help, though, since
> > > the compiler fails to compile the code in the first place because, as
> > > it says, it doesn't like '-march=r3000' without '-mfp32'.
> > 
> >  Indeed I got that confused, even though there is supposed to be a
> > mention in the compiler's diagnostics if a warning or error has
> > originated from an external tool invoked.  Sorry about that.  The
> > message itself could well have come from the assembler though as the
> > same options are accepted by that tool.
> > 
> >  And indeed I have just guessed what the cause might be, that is the
> > compiler must have been configured with an explicit `--with-fp-32=xx' or
> > `--with-fp-32=64' option.
> 
> This is almost certainly using --with-fp-32=xx
> 
> >  Here's quite an extensive analysis of what these options do and why:
> > <https://dmz-portal.mips.com/wiki/MIPS_O32_ABI_-
> > _FR0_and_FR1_Interlinking>
> > and I note that there is a mention of a setup which would have avoided
> > the situation we have now:
> > 
> > "Ideally any MIPS I arch/core will default to -mfp32 Any MIPS II ->
> > MIPS32r2 arch/core will default to -mfpxx. However, this should be
> > controlled via a configure time option to adjust the default ABI from
> > O32
> > FP32 to O32 FPXX (or O32 FP64 as needed). The new configure time option
> > is --with-fp-32=[32|xx|64] and this affects the FP mode only when
> > targetting the O32 ABI."
> > 
> > however this ideal arrangement, which I would expect a configuration
> > option like `--with-fp-32=from-isa' (and a corresponding `-mfp=from-isa'
> > compiler option) would handle, got lost/forgotten in the works somehow.
> 
> The intention of these comments was about vendor tool configurations that
> often do more option inference than standard tools. We could do a
> --with-fp-32=from-isa but that would not be used by debian anyway as it
> does not meet the goal of reliably generating FP compatible code. There are
> also subjective answers to the 'from-isa' question as mips32r1 could be
> fp32 or fpxx depending on what the toolchain owner wanted to achieve.
> 
> >  Matthew has been the expert in this area and might be able to add some
> > more -- Matthew?  Also shall we call it a compiler defect?  We seem to
> > be getting more and more toolchain configurations which break the
> > existing setups in a way requiring them to add more and more compiler
> > options to Makefiles for preexisting targets even though the Makefiles
> > were previously already prepared to choose the right compiler options
> > for the target.  This seems the wrong way to go to me as it's causing
> > people burden when upgrading their toolchains while it's supposed to be
> > seamless (it's one aspect of maintaining backwards compatibility).
> 
> I agree we don't want to take decisions lightly that end up breaking
> builds but we should not shy away from them.
> 
> In this specific case a kernel for a MIPS I arch is being built from a
> toolchain in a distribution that requires MIPS II as a minimum and is
> moving towards requiring MIPS32R2 as a minimum. The decisions made when
> configuring this toolchain are there to support the intended users so
> as we move forward there will be some things that stop working as
> before (by default).
> 
> Given that then I think this is reasonable fall-out. The options are:
> Build this configuration of the kernel using an older distribution,
> Build a custom toolchain, Update the kernel to cope with this new
> configuration.
> 
> In general I think it is reasonable for people to request continued
> support for architectures or configurations that are older and have a
> smaller user-base but such users may have to invest a little more effort
> in configuring as focus is moved to support the wider used variants.
> 
> We are also talking about a relatively new feature in the kernel and I
> would go so far as to say this is a great example of why trying to
> support every MIPS variant is futile; even those with the greatest
> knowledge of the architecture, toolchain and history cannot make it
> work in every case.
> 
> An even simpler solution to this, and better for long term support, is
> to disable the vDSO feature for architectures older than 'x' where we
> could choose MIPS II or even MIPS32R2 depending on how much simpler
> we would like life to be.
> 
> >  Meanwhile your proposal may be indeed the way to go, or perhaps we
> > could use a substitution rule like:
> > 
> > 	$(patsubst -march=%,-mfp32,$(filter -march=r3%,$(KBUILD_CFLAGS)))
> > 
> > -- see the change below, it seems to work for me and the option is
> > indeed passed with a `-march=r3000' build (as would with a `-
> > march=r3900' build) and suppressed otherwise (such as with a `-
> > march=mips32r2' build).
> 
> Does the kernel build both hard and soft float variants of the vDSO? When
> originally discussing the design for it I don't think it was supposed to.
> 
> I believe the .MIPS.abiflags section for the vDSO is overwritten to
> claim it is an 'any' FP ABI so that it is compatible with all possible
> userland variants so it better not contain any floating point instructions.
> 
> I'd therefore suggest using -msoft-float unconditionally is the way to go
> when building the vDSO even if it gets disabled for MIPS I completely as
> well.
> 
> >  I have tried building `arch/mips/vdso/gettimeofday.s' to see how the
> > module settings are chosen by the compiler in my successful build, and
> > that produced an unexpected result in that the rule for .s targets is
> > evidently broken for VDSO as different compiler flags were used from
> > ones used for .o targets -- essentially the regular flags rather than
> > the special VDSO ones (no `-fPIC', etc.).  So that looks like a bug to
> > me to be fixed too.
> > 
> >  So instead I just checked the ABI annotation of a good .o object with
> > `readelf':
> > 
> > $ readelf -A arch/mips/vdso/gettimeofday.o Attribute Section: gnu File
> > Attributes
> >   Tag_GNU_MIPS_ABI_FP: Hard float (double precision)
> > 
> > MIPS ABI Flags Version: 0
> > 
> > ISA: MIPS1
> > GPR size: 32
> > CPR1 size: 32
> > CPR2 size: 0
> > FP ABI: Hard float (double precision)
> > ISA Extension: None
> > ASEs:
> > 	None
> > FLAGS 1: 00000000
> > FLAGS 2: 00000000
> > $
> > 
> > and I think this would be good to keep.  Obviously this will prevent `-
> > mfp64' executables or DSOs from being loaded together with VDSO, but
> > they are incompatible with the 32-bit FPU MIPS I processors have anyway,
> > so no change in the semantics there.
> > 
> >  On the other hand there's still the issue what the compiler default
> > will be for o32 in a non-r3k build -- which could be any of `-mfp32', `-
> > mfpxx'
> > or `-mfp64'.  Requesting `-mfpxx' should allow the greatest flexibility,
> > but may not be supported as older compilers did not have it, so this
> > would have to be added conditionally only, letting those older compilers
> > retain their long-established `-mfp32' default.  I have an idea how to
> > implement this part if we agree this is indeed the right direction.
> 
> As above, unless absolutely critical to have floating point code then
> the vDSO should just avoid all FP related issues and build soft-float.
> 

FWIW, my logic was quite simple: The rest of the kernel builds with
-msoft-float, thus vDSO should do the same. Of course, I don't know the
entire context, so there may well be a reason to handle it differently
than the rest of the kernel.

If building with -msoft-float is not acceptable for vDSO, maybe there
is a means to detect if the toolchain supports the problematic case,
ie '-march=r3000' without '-mfp32', and not build vDSO in that case ?

Anyway, isn't the kernel supposed to not use floating point operations
in the first place ? Is this different for vDSO ?

Thanks,
Guenter

> Thanks,
> Matthew
> 
> >  For the record: `-mfp32' has been there since forever, my little
> > research indicating this commit:
> > 
> > Wed Jan 15 06:22:34 1992  Michael Meissner  (meissner at osf.org)
> > 
> > 	* mips.h (MIPS_VERSION): Bump Meissner version # to 8.
> > 	(fcmp_op): Add function declaration.
> > 	(TARGET_FLOAT64): Support for -mfp64 switch to support the R4000 FR
> > 	psw bit, which doubles the number of floating point registers.
> > 	(TARGET_SWITCHES): Likewise.
> > 	(HARD_REGNO_NREGS): Likewise.
> > 	(CLASS_FCMP_OP): New bit for classifing floating point compares.
> > 	(PREDICATE_CODES): Add cmp2_op and fcmp_op support.
> > [...]
> > 
> > which corresponds to GCC 2.0 released roughly a month afterwards,
> > although the exact diff seems to have been lost in the mist of history.
> > The oldest version of `mips.h' in the modern GCC repository dates back
> > to r297 from
> > 11 Feb 1992 where it already contains `-mfp32' support; obviously a sign
> > of an inaccurate import of those old changes.  Anyway the option is
> > surely always safe to use nowadays.
> > 
> >   Maciej
> > 
> > linux-mips-r3k-vdso-fp32.diff
> > Index: linux-sfr-decstation/arch/mips/vdso/Makefile
> > ===================================================================
> > --- linux-sfr-decstation.orig/arch/mips/vdso/Makefile	2016-10-23
> > 06:15:11.000000000 +0100
> > +++ linux-sfr-decstation/arch/mips/vdso/Makefile	2016-11-03
> > 22:44:31.752065000 +0000
> > @@ -6,7 +6,8 @@ ccflags-vdso := \
> >  	$(filter -I%,$(KBUILD_CFLAGS)) \
> >  	$(filter -E%,$(KBUILD_CFLAGS)) \
> >  	$(filter -mmicromips,$(KBUILD_CFLAGS)) \
> > -	$(filter -march=%,$(KBUILD_CFLAGS))
> > +	$(filter -march=%,$(KBUILD_CFLAGS)) \
> > +	$(patsubst -march=%,-mfp32,$(filter -march=r3%,$(KBUILD_CFLAGS)))
> >  cflags-vdso := $(ccflags-vdso) \
> >  	$(filter -W%,$(filter-out -Wa$(comma)%,$(KBUILD_CFLAGS))) \
> >  	-O2 -g -fPIC -fno-strict-aliasing -fno-common -fno-builtin -G 0 \

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