linux-kernel - Re: [kvm-unit-tests PATCH 0/3] Fix arm64 clang errors on fpu tests

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Message-ID: <CAJHc60wRgy2Hp6ZKAAmfH0sNFOFgvRXr_8-YuNf9+HV9-xwWDQ@mail.gmail.com>
Date: Tue, 22 Oct 2024 13:45:40 -0700
From: Raghavendra Rao Ananta <rananta@...gle.com>
To: Andrew Jones <andrew.jones@...ux.dev>
Cc: Subhasish Ghosh <subhasish.ghosh@....com>, Joey Gouly <joey.gouly@....com>, 
	Oliver Upton <oliver.upton@...ux.dev>, Marc Zyngier <maz@...nel.org>, 
	linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev, 
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [kvm-unit-tests PATCH 0/3] Fix arm64 clang errors on fpu tests

Hi Andrew,

On Tue, Oct 22, 2024 at 6:10 AM Andrew Jones <andrew.jones@...ux.dev> wrote:
>
> With clang 18.1.8 (Fedora 18.1.8-1.fc40) I get a bunch of errors like
> these
>
>     arm/fpu.c:281:3: error: instruction requires: fp-armv8
>
> I used my cross-clang series[1] and configured with
>
>     ./configure --arch=arm64 --cc=clang --cflags='--target=aarch64' --cross-prefix=aarch64-linux-gnu-
>
> [1] https://lore.kernel.org/all/20240911091406.134240-7-andrew.jones@linux.dev/
>
> Thanks,
> drew

I was able to reproduce the errors by pointing to a newer clang (20)
and applying your series.
I think we see the errors because llvm decided to disable loads and
stores on FP registers with "-mgeneral-regs-only" [1]. Explicitly
adding ".arch_extension fp" for the fp_reg_{read,write}() helped with
the build:

diff --git a/arm/fpu.c b/arm/fpu.c
index 6b0411d3..f44ed82a 100644
--- a/arm/fpu.c
+++ b/arm/fpu.c
@@ -38,7 +38,8 @@ static inline bool arch_collect_entropy(uint64_t *random)
 #define fpu_reg_read(val)                              \
 ({                                                     \
        uint64_t *__val = (val);                        \
-       asm volatile("stp q0, q1, [%0], #32\n\t"        \
+       asm volatile(".arch_extension fp\n"             \
+                    "stp q0, q1, [%0], #32\n\t"        \
                     "stp q2, q3, [%0], #32\n\t"        \
                     "stp q4, q5, [%0], #32\n\t"        \
                     "stp q6, q7, [%0], #32\n\t"        \
@@ -71,7 +72,8 @@ static inline bool arch_collect_entropy(uint64_t *random)
 #define fpu_reg_write(val)                             \
 do {                                                   \
        uint64_t *__val = (val);                        \
-       asm volatile("ldp q0, q1, [%0], #32\n\t"        \
+       asm volatile(".arch_extension fp\n"             \
+                    "ldp q0, q1, [%0], #32\n\t"        \
                     "ldp q2, q3, [%0], #32\n\t"        \
                     "ldp q4, q5, [%0], #32\n\t"        \
                     "ldp q6, q7, [%0], #32\n\t"        \

If you are fine with this, I can push it as a separate patch in v2.

Thank you.
Raghavendra

[1]: https://github.com/llvm/llvm-project/pull/77817