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Message-ID: <20240906190655.2777023-1-Jason@zx2c4.com>
Date: Fri, 6 Sep 2024 21:06:55 +0200
From: "Jason A. Donenfeld" <Jason@...c4.com>
To: tglx@...utronix.de,
mingo@...hat.com,
bp@...en8.de,
dave.hansen@...ux.intel.com,
x86@...nel.org,
linux-kernel@...r.kernel.org
Cc: "Jason A. Donenfeld" <Jason@...c4.com>
Subject: [PATCH tip] x86: vdso: Compute timens page offset statically
The expression `((void *)&__timens_vdso_data - (void *)&__vdso_data)`
seems harmless, but it actually results in quite a bit of code and two
jumps, in a place that's supposed to be somewhat lean on code. The value
of that calculation is always 3*PAGE_SIZE, as it turns out. Changing it
to that results in a more modest cmov instruction being emitted. It also
makes it a bit more clear what's happening.
To accomplish this, define offset macros in vvar.h, which can be shared
by C code and by the linker script that decides where these pages will
actually go.
Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
---
arch/x86/entry/vdso/vdso-layout.lds.S | 11 +++++------
arch/x86/include/asm/vdso/getrandom.h | 2 +-
arch/x86/include/asm/vvar.h | 5 +++++
3 files changed, 11 insertions(+), 7 deletions(-)
diff --git a/arch/x86/entry/vdso/vdso-layout.lds.S b/arch/x86/entry/vdso/vdso-layout.lds.S
index bafa73f09e92..ddd6999b6946 100644
--- a/arch/x86/entry/vdso/vdso-layout.lds.S
+++ b/arch/x86/entry/vdso/vdso-layout.lds.S
@@ -16,17 +16,16 @@ SECTIONS
* segment.
*/
- vvar_start = . - 4 * PAGE_SIZE;
- vvar_page = vvar_start;
-
/* Place all vvars at the offsets in asm/vvar.h. */
#define EMIT_VVAR(name, offset) vvar_ ## name = vvar_page + offset;
#include <asm/vvar.h>
#undef EMIT_VVAR
- pvclock_page = vvar_start + PAGE_SIZE;
- hvclock_page = vvar_start + 2 * PAGE_SIZE;
- timens_page = vvar_start + 3 * PAGE_SIZE;
+ vvar_start = . - 4 * PAGE_SIZE;
+ vvar_page = vvar_start + VVAR_PAGE_OFFSET * PAGE_SIZE;
+ pvclock_page = vvar_start + PVCLOCK_PAGE_OFFSET * PAGE_SIZE;
+ hvclock_page = vvar_start + HVCLOCK_PAGE_OFFSET * PAGE_SIZE;
+ timens_page = vvar_start + TIMENS_PAGE_OFFSET * PAGE_SIZE;
#undef _ASM_X86_VVAR_H
/* Place all vvars in timens too at the offsets in asm/vvar.h. */
diff --git a/arch/x86/include/asm/vdso/getrandom.h b/arch/x86/include/asm/vdso/getrandom.h
index b96e674cafde..3569fe95aa4e 100644
--- a/arch/x86/include/asm/vdso/getrandom.h
+++ b/arch/x86/include/asm/vdso/getrandom.h
@@ -33,7 +33,7 @@ static __always_inline ssize_t getrandom_syscall(void *buffer, size_t len, unsig
static __always_inline const struct vdso_rng_data *__arch_get_vdso_rng_data(void)
{
if (IS_ENABLED(CONFIG_TIME_NS) && __vdso_data->clock_mode == VDSO_CLOCKMODE_TIMENS)
- return (void *)&__vdso_rng_data + ((void *)&__timens_vdso_data - (void *)&__vdso_data);
+ return (void *)&__vdso_rng_data + (TIMENS_PAGE_OFFSET << CONFIG_PAGE_SHIFT);
return &__vdso_rng_data;
}
diff --git a/arch/x86/include/asm/vvar.h b/arch/x86/include/asm/vvar.h
index 9d9af37f7cab..d2a2ffa72909 100644
--- a/arch/x86/include/asm/vvar.h
+++ b/arch/x86/include/asm/vvar.h
@@ -19,6 +19,11 @@
#ifndef _ASM_X86_VVAR_H
#define _ASM_X86_VVAR_H
+#define VVAR_PAGE_OFFSET 0
+#define PVCLOCK_PAGE_OFFSET 1
+#define HVCLOCK_PAGE_OFFSET 2
+#define TIMENS_PAGE_OFFSET 3
+
#ifdef EMIT_VVAR
/*
* EMIT_VVAR() is used by the kernel linker script to put vvars in the
--
2.46.0
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