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Message-ID: <mhng-6127df88-9768-4f9d-8d78-a1aeb4213451@palmer-ri-x1c9>
Date: Thu, 18 Jan 2024 14:20:42 -0800 (PST)
From: Palmer Dabbelt <palmer@...belt.com>
To: Conor Dooley <conor@...nel.org>
CC: charlie@...osinc.com, Paul Walmsley <paul.walmsley@...ive.com>,
aou@...s.berkeley.edu, linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] riscv: lib: Support csum on GCC <11
On Thu, 18 Jan 2024 14:05:44 PST (-0800), Conor Dooley wrote:
> On Thu, Jan 18, 2024 at 01:53:59PM -0800, Charlie Jenkins wrote:
>> The OutputOperands field for asm goto statements is only supported
>> starting from GCC 11. Split the asm goto to remove the use of this
>> feature.
>>
>> Signed-off-by: Charlie Jenkins <charlie@...osinc.com>
>> Fixes: a04c192eabfb ("riscv: Add checksum library")
>> ---
>> The OutputOperands field for asm goto statements is only supported
>> starting from GCC 11. Split the asm goto to remove the use of this
>> feature.
>
> Maybe this is a super naive question, but is it possible to just not
> use the custom csum code for gcc older than 11?
Charlie and I were talking, these old GCC versions also don't support
ZBB. So I think we can get away with something like
diff --git a/arch/riscv/lib/csum.c b/arch/riscv/lib/csum.c
index 06ce8e7250d9..17f883b612c9 100644
--- a/arch/riscv/lib/csum.c
+++ b/arch/riscv/lib/csum.c
@@ -158,10 +158,16 @@ do_csum_with_alignment(const unsigned char *buff, int len)
/*
* Zbb support saves 6 instructions, so not worth checking without
- * alternatives if supported
+ * alternatives if supported.
+ *
+ * Note that we pull the check for ZBB into the preprocessor proper
+ * here, as otherwise GCC will attempt to compile the code inside the
+ * "if (IS_ENABLED(ZBB)" block which fails because GCC10 doesn't
+ * support ASM goto output operands. GCC 10 also doesn't support ZBB,
+ * so we're safe with that check here.
*/
- if (IS_ENABLED(CONFIG_RISCV_ISA_ZBB) &&
- IS_ENABLED(CONFIG_RISCV_ALTERNATIVE)) {
+#if defined(CONFIG_RISCV_ISA_ZBB)
+ if (IS_ENABLED(CONFIG_RISCV_ALTERNATIVE)) {
unsigned long fold_temp;
/*
@@ -213,6 +219,7 @@ do_csum_with_alignment(const unsigned char *buff, int len)
end:
return csum >> 16;
}
+#endif /*CONFIG_RISCV_ISA_ZBB*/
no_zbb:
#ifndef CONFIG_32BIT
csum += ror64(csum, 32);
@@ -244,10 +251,11 @@ do_csum_no_alignment(const unsigned char *buff, int len)
/*
* Zbb support saves 6 instructions, so not worth checking without
- * alternatives if supported
+ * alternatives if supported. See above for the ZBB preprocessor
+ * check.
*/
- if (IS_ENABLED(CONFIG_RISCV_ISA_ZBB) &&
- IS_ENABLED(CONFIG_RISCV_ALTERNATIVE)) {
+#if defined(CONFIG_RISCV_ISA_ZBB)
+ if (IS_ENABLED(CONFIG_RISCV_ALTERNATIVE)) {
unsigned long fold_temp;
/*
@@ -287,6 +295,7 @@ do_csum_no_alignment(const unsigned char *buff, int len)
#endif /* !CONFIG_32BIT */
return csum >> 16;
}
+#endif /*CONFIG_RISCV_ISA_ZBB*/
no_zbb:
#ifndef CONFIG_32BIT
csum += ror64(csum, 32);
which is building for me on GCC-10/defconfig.
>
>> ---
>> arch/riscv/lib/csum.c | 42 ++++++++++++++++++++++++++++++------------
>> 1 file changed, 30 insertions(+), 12 deletions(-)
>>
>> diff --git a/arch/riscv/lib/csum.c b/arch/riscv/lib/csum.c
>> index 06ce8e7250d9..23be289f52b6 100644
>> --- a/arch/riscv/lib/csum.c
>> +++ b/arch/riscv/lib/csum.c
>> @@ -177,22 +177,35 @@ do_csum_with_alignment(const unsigned char *buff, int len)
>> : no_zbb);
>>
>> #ifdef CONFIG_32BIT
>> - asm_volatile_goto(".option push \n\
>> + /*
>> + * OutputOperands in asm goto is not supported until GCC 11, so
>> + * this asm has to be split to be compatible.
>> + */
>> + asm (".option push \n\
>> .option arch,+zbb \n\
>> rori %[fold_temp], %[csum], 16 \n\
>> andi %[offset], %[offset], 1 \n\
>> add %[csum], %[fold_temp], %[csum] \n\
>> - beq %[offset], zero, %l[end] \n\
>> - rev8 %[csum], %[csum] \n\
>> .option pop"
>> : [csum] "+r" (csum), [fold_temp] "=&r" (fold_temp)
>> - : [offset] "r" (offset)
>> - :
>> - : end);
>> + : [offset] "r" (offset));
>> +
>> + if (offset == 0)
>> + goto end;
>> +
>> + asm (".option push \n\
>> + .option arch, +zbb \n\
>> + rev8 %[csum], %[csum] \n\
>> + .option pop"
>> + : [csum] "+r" (csum));
>>
>> return (unsigned short)csum;
>> #else /* !CONFIG_32BIT */
>> - asm_volatile_goto(".option push \n\
>> + /*
>> + * OutputOperands in asm goto is not supported until GCC 11, so
>> + * this asm has to be split to be compatible.
>> + */
>> + asm (".option push \n\
>> .option arch,+zbb \n\
>> rori %[fold_temp], %[csum], 32 \n\
>> add %[csum], %[fold_temp], %[csum] \n\
>> @@ -200,13 +213,18 @@ do_csum_with_alignment(const unsigned char *buff, int len)
>> roriw %[fold_temp], %[csum], 16 \n\
>> addw %[csum], %[fold_temp], %[csum] \n\
>> andi %[offset], %[offset], 1 \n\
>> - beq %[offset], zero, %l[end] \n\
>> - rev8 %[csum], %[csum] \n\
>> .option pop"
>> : [csum] "+r" (csum), [fold_temp] "=&r" (fold_temp)
>> - : [offset] "r" (offset)
>> - :
>> - : end);
>> + : [offset] "r" (offset));
>> +
>> + if (offset == 0)
>> + goto end;
>> +
>> + asm (".option push \n\
>> + .option arch, +zbb \n\
>> + rev8 %[csum], %[csum] \n\
>> + .option pop"
>> + : [csum] "+r" (csum));
>>
>> return (csum << 16) >> 48;
>> #endif /* !CONFIG_32BIT */
>>
>> ---
>> base-commit: 080c4324fa5e81ff3780206a138223abfb57a68e
>> change-id: 20240118-csum_remove_output_operands_asm_goto-49922c141ce7
>> --
>> - Charlie
>>
>>
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