On Wed, Oct 02, 2024 at 10:39:29AM +0200, Christophe Leroy wrote:
> VDSO time functions do not call any other function, so they don't
> need to save/restore LR. However, retrieving the address of VDSO data
> page requires using LR hence saving then restoring it, which can be
> heavy on some CPUs. On the other hand, VDSO functions on powerpc are
> not standard functions and require a wrapper function to call C VDSO
> functions. And that wrapper has to save and restore LR in order to
> call the C VDSO function, so retrieving VDSO data page address in that
> wrapper doesn't require additional save/restore of LR.
>
> For random VDSO functions it is a bit different. Because the function
> calls __arch_chacha20_blocks_nostack(), it saves and restores LR.
> Retrieving VDSO data page address can then be done there without
> additional save/restore of LR.
>
> So lets implement __arch_get_vdso_rng_data() and simplify the wrapper.
>
> It starts paving the way for the day powerpc will implement a more
> standard ABI for VDSO functions.
>
> Signed-off-by: Christophe Leroy <christophe.le...@csgroup.eu>
> ---
> arch/powerpc/include/asm/vdso/getrandom.h | 15 +++++++++++++--
> arch/powerpc/kernel/asm-offsets.c | 1 -
> arch/powerpc/kernel/vdso/getrandom.S | 1 -
> arch/powerpc/kernel/vdso/vgetrandom.c | 4 ++--
> 4 files changed, 15 insertions(+), 6 deletions(-)
>
> diff --git a/arch/powerpc/include/asm/vdso/getrandom.h
> b/arch/powerpc/include/asm/vdso/getrandom.h
> index 501d6bb14e8a..4302e7c67aa5 100644
> --- a/arch/powerpc/include/asm/vdso/getrandom.h
> +++ b/arch/powerpc/include/asm/vdso/getrandom.h
> @@ -7,6 +7,8 @@
>
> #ifndef __ASSEMBLY__
>
> +#include <asm/vdso_datapage.h>
> +
> static __always_inline int do_syscall_3(const unsigned long _r0, const
> unsigned long _r3,
> const unsigned long _r4, const unsigned
> long _r5)
> {
> @@ -43,11 +45,20 @@ static __always_inline ssize_t getrandom_syscall(void
> *buffer, size_t len, unsig
>
> static __always_inline struct vdso_rng_data *__arch_get_vdso_rng_data(void)
> {
> - return NULL;
> + struct vdso_arch_data *data;
> +
> + asm(
> + " bcl 20, 31, .+4\n"
> + "0: mflr %0\n"
> + " addis %0, %0, (_vdso_datapage - 0b)@ha\n"
> + " addi %0, %0, (_vdso_datapage - 0b)@l\n"
> + : "=r" (data) :: "lr");
> +
> + return &data->rng_data;
> }
Did you also try something like this:
extern struct vdso_arch_data _vdso_datapage
__attribute__((visibility("hidden")));
static __always_inline struct vdso_rng_data *__arch_get_vdso_rng_data(void)
{
return &_vdso_datapage.rng_data;
}
Not knowing much about ppc asm the resulting assembly looks simpler.
And it would be more in line with what other archs are doing.
> [..]
