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The rdfsbase() and rdgsbase() helpers currently include a "memory" clobber in their inline assembly definitions. However, the RDFSBASE and RDGSBASE instructions only read the FS/GS base MSRs into a general-purpose register and do not access memory. The "memory" clobber, which acts as a compiler barrier and may inhibit optimization, is therefore unnecessary. The "memory" clobber was historically used as a scheduling constraint to prevent the compiler from moving the instructions before preceding segment register loads. This is not required because both the segment register loads and the RDFSBASE/RDGSBASE accessors are implemented with `asm volatile`, which already prevents reordering between them. No functional change intended. Signed-off-by: Uros Bizjak <ubizjak@gmail.com> Signed-off-by: Ingo Molnar <mingo@kernel.org> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: H. Peter Anvin <hpa@zytor.com> Link: https://patch.msgid.link/20260330055823.5793-1-ubizjak@gmail.com
86 lines
2.0 KiB
C
86 lines
2.0 KiB
C
/* SPDX-License-Identifier: GPL-2.0 */
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#ifndef _ASM_FSGSBASE_H
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#define _ASM_FSGSBASE_H
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#ifndef __ASSEMBLER__
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#ifdef CONFIG_X86_64
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#include <asm/msr.h>
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/*
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* Read/write a task's FSBASE or GSBASE. This returns the value that
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* the FS/GS base would have (if the task were to be resumed). These
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* work on the current task or on a non-running (typically stopped
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* ptrace child) task.
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*/
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extern unsigned long x86_fsbase_read_task(struct task_struct *task);
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extern unsigned long x86_gsbase_read_task(struct task_struct *task);
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extern void x86_fsbase_write_task(struct task_struct *task, unsigned long fsbase);
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extern void x86_gsbase_write_task(struct task_struct *task, unsigned long gsbase);
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/* Must be protected by X86_FEATURE_FSGSBASE check. */
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static __always_inline unsigned long rdfsbase(void)
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{
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unsigned long fsbase;
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asm volatile("rdfsbase %0" : "=r" (fsbase));
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return fsbase;
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}
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static __always_inline unsigned long rdgsbase(void)
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{
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unsigned long gsbase;
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asm volatile("rdgsbase %0" : "=r" (gsbase));
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return gsbase;
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}
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static __always_inline void wrfsbase(unsigned long fsbase)
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{
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asm volatile("wrfsbase %0" :: "r" (fsbase) : "memory");
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}
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static __always_inline void wrgsbase(unsigned long gsbase)
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{
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asm volatile("wrgsbase %0" :: "r" (gsbase) : "memory");
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}
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#include <asm/cpufeature.h>
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/* Helper functions for reading/writing FS/GS base */
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static inline unsigned long x86_fsbase_read_cpu(void)
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{
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unsigned long fsbase;
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if (boot_cpu_has(X86_FEATURE_FSGSBASE))
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fsbase = rdfsbase();
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else
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rdmsrq(MSR_FS_BASE, fsbase);
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return fsbase;
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}
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static inline void x86_fsbase_write_cpu(unsigned long fsbase)
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{
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if (boot_cpu_has(X86_FEATURE_FSGSBASE))
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wrfsbase(fsbase);
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else
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wrmsrq(MSR_FS_BASE, fsbase);
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}
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extern unsigned long x86_gsbase_read_cpu_inactive(void);
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extern void x86_gsbase_write_cpu_inactive(unsigned long gsbase);
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extern unsigned long x86_fsgsbase_read_task(struct task_struct *task,
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unsigned short selector);
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#endif /* CONFIG_X86_64 */
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#endif /* __ASSEMBLER__ */
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#endif /* _ASM_FSGSBASE_H */
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