Re: [PATCH] RISC-V: Mask out the F extension on systems without D
From: Palmer Dabbelt
Date: Tue Aug 28 2018 - 12:51:29 EST
On Tue, 28 Aug 2018 00:10:32 PDT (-0700), alankao@xxxxxxxxxxxxx wrote:
Hi Palmer,
On Mon, Aug 27, 2018 at 03:03:52PM -0700, Palmer Dabbelt wrote:
The RISC-V Linux port doesn't support systems that have the F extension
but don't have the D extension -- we actually don't support systems
without D either, but Alan's patch set is rectifying that soon. For now
I think we can leave this in a semi-broken state and just wait for
Alan's patch set to get merged for proper non-FPU support -- the patch
set is starting to look good, so doing something in-between doesn't seem
like it's worth the work.
I don't think it's worth fretting about support for systems with F but
not D for now: our glibc ABIs are IMAC and IMAFDC so they probably won't
end up being popular. We can always extend this in the future.
CC: Alan Kao <alankao@xxxxxxxxxxxxx>
Signed-off-by: Palmer Dabbelt <palmer@xxxxxxxxxx>
---
arch/riscv/kernel/cpufeature.c | 7 +++++++
1 file changed, 7 insertions(+)
diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
index 17011a870044..652d102ffa06 100644
--- a/arch/riscv/kernel/cpufeature.c
+++ b/arch/riscv/kernel/cpufeature.c
@@ -57,5 +57,12 @@ void riscv_fill_hwcap(void)
for (i = 0; i < strlen(isa); ++i)
elf_hwcap |= isa2hwcap[(unsigned char)(isa[i])];
+ /* We don't support systems with F but without D, so mask those out
+ * here. */
+ if ((elf_hwcap & COMPAT_HWCAP_ISA_F) && !(elf_hwcap & COMPAT_HWCAP_ISA_D)) {
+ pr_info("This kernel does not support systems with F but not D");
+ elf_hwcap &= ~COMPAT_HWCAP_ISA_F;
+ }
+
The commit message does address the problem and this patch does provide checks
and helpful information to users, but I wonder if we really need this patch, for
two reasons:
* Just as you mentioned, current glibc ABI does not support such a thing as
IMAFC, so probably no one has had trouble with this. To be honest, I suppose
that anybody (RISC-V enthusiasts or vendors) who really need F-only support
in kernel should get themself involved in the development by sending patches
to improve.
* There are corner cases to let a F-only machine to pass the check in this
patch. For instance, a vendor decides to name her extension ISA as doom,
and supports single-precision FP only, so her ISA string would be
IMAFCXdoom.
The variable elf_hwcap is calculated at the loop in line 57,58, the 'd'
from Xdoom would bypass the check, while the underlying machine does not
support double-precision FP.
Ah, yes, that makes sense. I'd go the other way here and just be strict about
parsing the ISA string: it's defined to be listed in a particular order, so we
should really only be accepting legal ISA strings.
I'll submit a second patch to fix this behavior.
pr_info("elf_hwcap is 0x%lx", elf_hwcap);
}
--
2.16.4
I don't know if the reasons make sense to you, but anyway that's all I
would like to say about this patch.
Alan