Re: [PATCH v4 3/5] Cleanup ISA string setting

[Palmer Dabbelt]

On Mon, 20 Aug 2018 19:47:28 PDT (-0700), alankao@xxxxxxxxxxxxx wrote:
> On Mon, Aug 20, 2018 at 03:22:55PM -0700, Palmer Dabbelt wrote:
> > On Tue, 07 Aug 2018 20:24:43 PDT (-0700), alankao@xxxxxxxxxxxxx wrote:
> > >Just a side note: (Assume that atomic and compressed is on)
> > >
> > >Before this patch, assembler was always given the riscv64imafdc
> > >MARCH string because there are fld/fsd's in entry.S; compiler was
> > >always given riscv64imac because kernel doesn't need floating point
> > >code generation.  After this, the MARCH string in AFLAGS and CFLAGS
> > >become the same.
> > >
> > >Signed-off-by: Alan Kao <alankao@xxxxxxxxxxxxx>
> > >Cc: Greentime Hu <greentime@xxxxxxxxxxxxx>
> > >Cc: Vincent Chen <vincentc@xxxxxxxxxxxxx>
> > >Cc: Zong Li <zong@xxxxxxxxxxxxx>
> > >Cc: Nick Hu <nickhu@xxxxxxxxxxxxx>
> > >Reviewed-by: Christoph Hellwig <hch@xxxxxx>
> > >---
> > > arch/riscv/Makefile | 19 ++++++++-----------
> > > 1 file changed, 8 insertions(+), 11 deletions(-)
> > >
> > >diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
> > >index 6d4a5f6c3f4f..e0fe6790624f 100644
> > >--- a/arch/riscv/Makefile
> > >+++ b/arch/riscv/Makefile
> > >@@ -26,7 +26,6 @@ ifeq ($(CONFIG_ARCH_RV64I),y)
> > >
> > > 	KBUILD_CFLAGS += -mabi=lp64
> > > 	KBUILD_AFLAGS += -mabi=lp64
> > >-	KBUILD_MARCH = rv64im
> > > 	LDFLAGS += -melf64lriscv
> > > else
> > > 	BITS := 32
> > >@@ -34,22 +33,20 @@ else
> > >
> > > 	KBUILD_CFLAGS += -mabi=ilp32
> > > 	KBUILD_AFLAGS += -mabi=ilp32
> > >-	KBUILD_MARCH = rv32im
> > > 	LDFLAGS += -melf32lriscv
> > > endif
> > >
> > > KBUILD_CFLAGS += -Wall
> > >
> > >-ifeq ($(CONFIG_RISCV_ISA_A),y)
> > >-	KBUILD_ARCH_A = a
> > >-endif
> > >-ifeq ($(CONFIG_RISCV_ISA_C),y)
> > >-	KBUILD_ARCH_C = c
> > >-endif
> > >-
> > >-KBUILD_AFLAGS += -march=$(KBUILD_MARCH)$(KBUILD_ARCH_A)fd$(KBUILD_ARCH_C)
> > >+# ISA string setting
> > >+riscv-march-$(CONFIG_ARCH_RV32I)	:= rv32im
> > >+riscv-march-$(CONFIG_ARCH_RV64I)	:= rv64im
> > >+riscv-march-$(CONFIG_RISCV_ISA_A)	:= $(riscv-march-y)a
> > >+riscv-march-y				:= $(riscv-march-y)fd
> > >+riscv-march-$(CONFIG_RISCV_ISA_C)	:= $(riscv-march-y)c
> > >+KBUILD_CFLAGS += -march=$(riscv-march-y)
> > >+KBUILD_AFLAGS += -march=$(riscv-march-y)
> > >
> > >-KBUILD_CFLAGS += -march=$(KBUILD_MARCH)$(KBUILD_ARCH_A)$(KBUILD_ARCH_C)
> >
> > We used to have "fd" disabled in KBUILD_CFLAGS because we wanted to ensure
> > that any use of floating-point types within the kernel would trigger the
> > inclusion of soft-float libraries rather than emitting hardware
> > floating-point instructions.  The worry here is that we'd end up corrupting
> > the user's floating-point state with the kernel accesses because of the lazy
> > save/restore stuff going on.
>
> Thanks for pointing that out.
>
> Just as you said, the use of KBUILD_CFLAGS=*fd* is based on the assumption that
> not a single floating-point instruction involves in the kernel, and that might
> be wrong.
>
> > I remember at some point it was illegal to use floating-point within the
> > kernel, but for some reason I thought that had changed.  I do see a handful
> > of references to "float" and "double" in the kernel source, and most of
> > references to kernel_fpu_begin() appear to be in SSE-specific stuff.  My one
> > worry are the usages in drivers/gpu/drm/amd/display/dc/calcs/dcn_calcs.c, as
> > I can't quickly demonstrate they won't happen.
>
> Empirically, this CFLAGS with *fd* did not cause any trouble to me, but of
> course this is not a good statement to support this patch.
>
> Meanwhile, I find a flaw in "[PATCH 4/5] Allow to Disable FPU Support."
> The purpose of this "[PATCH 3/5] Cleanup ISA String" was to make CONFIG_FPU
> a switch for the appearance of "fd" in both KBUILD_CFLAGS and KBUILD_ASFLAGS,
> but somehow the modification was forgotten.
>
> > If we do this I think we should at least ensure kernel_fpu_{begin,end}() do
> > the right thing for RISC-V.  I'd still feel safer telling the C compiler to
> > disallow floating-point, though, as I'm a bit paranoid that GCC might go and
> > emit a floating-point instruction even when it's not explicitly asked for.
> > I just asked Jim, who actually understands GCC, and he said that it'll spill
> > to floating-point registers if the cost function determines it's cheaper
> > than the stack.  While he thinks that's unlikely, I don't really want to
> > rely a GCC cost function for the correctness of our kernel port.
>
> The purpose of this whole patchset was to remove all floating-point-related
> logic in kernel space (while remainging FPU systems work as usual), so
> implementing the two APIs would be out of scope here.
>
> I suggest that, some people have to provide these APIs if they do need hardware
> floating-point calculation in kernel space (kernel or module) in the future.
> It seems that we don't need those for the kernel and any already supported
> hardware drivers at least for now.  Please correct me if my understanding is
> out-of-date.
>
> > I'd like to avoid having "-march=*fd*" in CFLAGS, unless someone can
> > convince me it's safe and that I'm just being too paranoid :).
>
> I will send a new version of the patchset, which will make sure that CFLAGS has
> no *fd* (3/5) and the CONFIG_FPU did remove *fd* from ASFLAGS (4/5).

Thanks!

> > > KBUILD_CFLAGS += -mno-save-restore
> > > KBUILD_CFLAGS += -DCONFIG_PAGE_OFFSET=$(CONFIG_PAGE_OFFSET)
>
> Thanks!
>
> Alan