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Message-ID: <mhng-efef4c03-1c58-4b86-9730-87959d73ee9e@palmerdabbelt-glaptop1>
Date: Wed, 09 Sep 2020 16:08:22 -0700 (PDT)
From: Palmer Dabbelt <palmerdabbelt@...gle.com>
To: dalias@...c.org, Arnd Bergmann <arnd@...db.de>
CC: musl@...ts.openwall.com
Subject: Re: riscv32 v2
On Wed, 09 Sep 2020 14:36:44 PDT (-0700), dalias@...c.org wrote:
> On Wed, Sep 09, 2020 at 02:28:55PM -0700, Palmer Dabbelt wrote:
>> On Wed, 09 Sep 2020 13:28:27 PDT (-0700), dalias@...c.org wrote:
>> >On Fri, Sep 04, 2020 at 01:48:19AM -0400, Stefan O'Rear wrote:
>> >>+#define VDSO_USEFUL
>> >>+/* We don't have a clock_gettime function.
>> >>+#define VDSO_CGT_SYM "__vdso_clock_gettime"
>> >>+#define VDSO_CGT_VER "LINUX_2.6" */
>> >>--
>> >>2.25.4
>> >>
>> >
>> >Is this correct? I see the comment is just copied from riscv64, but it
>> >seems wrong there, and here too. Also, is the vdso function named
>> >"clock_gettime" or "clock_gettime64" for riscv32? Or is there none at
>> >all and this macro just wrong?
>>
>> Looks like we don't have __vdso_clock_gettime on rv32 but we do have one on
>> rv64. glibc doesn't have the clock VDSO calls on rv32.
>>
>> I'm not opposed to adding some sort of clock-related VDSO calls on rv32, but it
>> looks like doing so will require some thought. Maybe it's best to wait on that
>> so we don't hold up the initial port?
>
> Possible addition of vdso clock_gettime isn't a blocker for moving
> forward with the musl port, but syscall_arch.h should accurately
> describe what's available and should not attempt to use vdso before
> it's a public kernel interface (e.g. resolving the question of what
> the function name will be). So I think it should be removed for now.
Sorry if that was confusing, but I definitely agree.
I guess my point was that the lack of VDSO clock functions on rv32 was probably
an oversight, but one that shouldn't block the port. We definitely can't just
make up a kernel interface, particularly as the reason we don't have these on
rv32 is because the generic versions of the functions we're using don't appear
to run on 32-bit targets.
That probably means there's some more subtle issue, though TBH I don't know
enough about the 64-bit-ification of time_t for it to just jump out at me. I
don't want to derail the thread too much, but I tried the obvious thing
diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index 33dde87218dd..1cf24a8f76c4 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -65,7 +65,7 @@ config RISCV
select HAVE_EBPF_JIT if MMU
select HAVE_FUTEX_CMPXCHG if FUTEX
select HAVE_GCC_PLUGINS
- select HAVE_GENERIC_VDSO if MMU && 64BIT
+ select HAVE_GENERIC_VDSO if MMU
select HAVE_PCI
select HAVE_PERF_EVENTS
select HAVE_PERF_REGS
diff --git a/arch/riscv/kernel/vdso/Makefile b/arch/riscv/kernel/vdso/Makefile
index 478e7338ddc1..10f7a07ce85a 100644
--- a/arch/riscv/kernel/vdso/Makefile
+++ b/arch/riscv/kernel/vdso/Makefile
@@ -7,9 +7,7 @@ ARCH_REL_TYPE_ABS := R_RISCV_32|R_RISCV_64|R_RISCV_JUMP_SLOT
include $(srctree)/lib/vdso/Makefile
# Symbols present in the vdso
vdso-syms = rt_sigreturn
-ifdef CONFIG_64BIT
vdso-syms += vgettimeofday
-endif
vdso-syms += getcpu
vdso-syms += flush_icache
and it doesn't build. I've added Arnd, who might have a better idea of what's
going on. Whatever happens, I think the best bet is to just drop the clock
functions (specifically __vdso_{clock_gettime,gettimeofday,clock_getres}) from
the rv32 port right now.
> But VDSO_USEFUL must be kept if we want to support the vdso icache
> flush function (is that actually supported on rv32 either? if not it
> should be made conditional on rv64 in src/linux/cache.c.
Yes, we have __vdso_flush_icache on rv32 and as far as I know it should work
fine (I guess QEMU isn't really going to find fence.i issues, but this
interface in particular is quite simple). There's no way to build a working
system without some kernel-based fence.i mechanism, and IIRC we added the VDSO
entry at the same time as the syscall so it should always work itself out
(though we do have the syscall-based fallback in glibc)). One of my working
directories reports
$ riscv64-linux-gnu-objdump -d arch/riscv/kernel/vdso/vdso.so
arch/riscv/kernel/vdso/vdso.so: file format elf32-littleriscv
Disassembly of section .text:
00000800 <__vdso_rt_sigreturn@@LINUX_4.15>:
800: 08b00893 li a7,139
804: 00000073 ecall
808: 0000 unimp
...
0000080c <__vdso_getcpu@@LINUX_4.15>:
80c: 0a800893 li a7,168
810: 00000073 ecall
814: 8082 ret
...
00000818 <__vdso_flush_icache@@LINUX_4.15>:
818: 10300893 li a7,259
81c: 00000073 ecall
820: 8082 ret
when built for rv32 (despite the rv64 objdump command).
>
> Rich
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