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x86/entry/64: Use 'xorl' for faster register clearing
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On some x86 CPU microarchitectures using 'xorq' to clear general-purpose
registers is slower than 'xorl'. As 'xorl' is sufficient to clear all
64 bits of these registers due to zero-extension [*], switch the x86
64-bit entry code to use 'xorl'.

No change in functionality and no change in code size.

[*] According to Intel 64 and IA-32 Architecture Software Developer's
    Manual, section 3.4.1.1, the result of 32-bit operands are "zero-
    extended to a 64-bit result in the destination general-purpose
    register." The AMD64 Architecture Programmer’s Manual Volume 3,
    Appendix B.1, describes the same behaviour.

Suggested-by: Denys Vlasenko <dvlasenk@redhat.com>
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20180214175924.23065-3-linux@dominikbrodowski.net
[ Improved on the changelog a bit. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>
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Dominik Brodowski authored and Ingo Molnar committed Feb 17, 2018
1 parent 9e809d1 commit ced5d0b
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Showing 2 changed files with 35 additions and 35 deletions.
16 changes: 8 additions & 8 deletions arch/x86/entry/calling.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -117,25 +117,25 @@ For 32-bit we have the following conventions - kernel is built with
pushq %rcx /* pt_regs->cx */
pushq \rax /* pt_regs->ax */
pushq %r8 /* pt_regs->r8 */
xorq %r8, %r8 /* nospec r8 */
xorl %r8d, %r8d /* nospec r8 */
pushq %r9 /* pt_regs->r9 */
xorq %r9, %r9 /* nospec r9 */
xorl %r9d, %r9d /* nospec r9 */
pushq %r10 /* pt_regs->r10 */
xorq %r10, %r10 /* nospec r10 */
xorl %r10d, %r10d /* nospec r10 */
pushq %r11 /* pt_regs->r11 */
xorq %r11, %r11 /* nospec r11*/
xorl %r11d, %r11d /* nospec r11*/
pushq %rbx /* pt_regs->rbx */
xorl %ebx, %ebx /* nospec rbx*/
pushq %rbp /* pt_regs->rbp */
xorl %ebp, %ebp /* nospec rbp*/
pushq %r12 /* pt_regs->r12 */
xorq %r12, %r12 /* nospec r12*/
xorl %r12d, %r12d /* nospec r12*/
pushq %r13 /* pt_regs->r13 */
xorq %r13, %r13 /* nospec r13*/
xorl %r13d, %r13d /* nospec r13*/
pushq %r14 /* pt_regs->r14 */
xorq %r14, %r14 /* nospec r14*/
xorl %r14d, %r14d /* nospec r14*/
pushq %r15 /* pt_regs->r15 */
xorq %r15, %r15 /* nospec r15*/
xorl %r15d, %r15d /* nospec r15*/
UNWIND_HINT_REGS
.if \save_ret
pushq %rsi /* return address on top of stack */
Expand Down
54 changes: 27 additions & 27 deletions arch/x86/entry/entry_64_compat.S
View file
Original file line number Diff line number Diff line change
Expand Up @@ -85,25 +85,25 @@ ENTRY(entry_SYSENTER_compat)
pushq %rcx /* pt_regs->cx */
pushq $-ENOSYS /* pt_regs->ax */
pushq $0 /* pt_regs->r8 = 0 */
xorq %r8, %r8 /* nospec r8 */
xorl %r8d, %r8d /* nospec r8 */
pushq $0 /* pt_regs->r9 = 0 */
xorq %r9, %r9 /* nospec r9 */
xorl %r9d, %r9d /* nospec r9 */
pushq $0 /* pt_regs->r10 = 0 */
xorq %r10, %r10 /* nospec r10 */
xorl %r10d, %r10d /* nospec r10 */
pushq $0 /* pt_regs->r11 = 0 */
xorq %r11, %r11 /* nospec r11 */
xorl %r11d, %r11d /* nospec r11 */
pushq %rbx /* pt_regs->rbx */
xorl %ebx, %ebx /* nospec rbx */
pushq %rbp /* pt_regs->rbp (will be overwritten) */
xorl %ebp, %ebp /* nospec rbp */
pushq $0 /* pt_regs->r12 = 0 */
xorq %r12, %r12 /* nospec r12 */
xorl %r12d, %r12d /* nospec r12 */
pushq $0 /* pt_regs->r13 = 0 */
xorq %r13, %r13 /* nospec r13 */
xorl %r13d, %r13d /* nospec r13 */
pushq $0 /* pt_regs->r14 = 0 */
xorq %r14, %r14 /* nospec r14 */
xorl %r14d, %r14d /* nospec r14 */
pushq $0 /* pt_regs->r15 = 0 */
xorq %r15, %r15 /* nospec r15 */
xorl %r15d, %r15d /* nospec r15 */
cld

/*
Expand Down Expand Up @@ -224,25 +224,25 @@ GLOBAL(entry_SYSCALL_compat_after_hwframe)
pushq %rbp /* pt_regs->cx (stashed in bp) */
pushq $-ENOSYS /* pt_regs->ax */
pushq $0 /* pt_regs->r8 = 0 */
xorq %r8, %r8 /* nospec r8 */
xorl %r8d, %r8d /* nospec r8 */
pushq $0 /* pt_regs->r9 = 0 */
xorq %r9, %r9 /* nospec r9 */
xorl %r9d, %r9d /* nospec r9 */
pushq $0 /* pt_regs->r10 = 0 */
xorq %r10, %r10 /* nospec r10 */
xorl %r10d, %r10d /* nospec r10 */
pushq $0 /* pt_regs->r11 = 0 */
xorq %r11, %r11 /* nospec r11 */
xorl %r11d, %r11d /* nospec r11 */
pushq %rbx /* pt_regs->rbx */
xorl %ebx, %ebx /* nospec rbx */
pushq %rbp /* pt_regs->rbp (will be overwritten) */
xorl %ebp, %ebp /* nospec rbp */
pushq $0 /* pt_regs->r12 = 0 */
xorq %r12, %r12 /* nospec r12 */
xorl %r12d, %r12d /* nospec r12 */
pushq $0 /* pt_regs->r13 = 0 */
xorq %r13, %r13 /* nospec r13 */
xorl %r13d, %r13d /* nospec r13 */
pushq $0 /* pt_regs->r14 = 0 */
xorq %r14, %r14 /* nospec r14 */
xorl %r14d, %r14d /* nospec r14 */
pushq $0 /* pt_regs->r15 = 0 */
xorq %r15, %r15 /* nospec r15 */
xorl %r15d, %r15d /* nospec r15 */

/*
* User mode is traced as though IRQs are on, and SYSENTER
Expand Down Expand Up @@ -298,9 +298,9 @@ sysret32_from_system_call:
*/
SWITCH_TO_USER_CR3_NOSTACK scratch_reg=%r8 scratch_reg2=%r9

xorq %r8, %r8
xorq %r9, %r9
xorq %r10, %r10
xorl %r8d, %r8d
xorl %r9d, %r9d
xorl %r10d, %r10d
swapgs
sysretl
END(entry_SYSCALL_compat)
Expand Down Expand Up @@ -358,25 +358,25 @@ ENTRY(entry_INT80_compat)
pushq %rcx /* pt_regs->cx */
pushq $-ENOSYS /* pt_regs->ax */
pushq $0 /* pt_regs->r8 = 0 */
xorq %r8, %r8 /* nospec r8 */
xorl %r8d, %r8d /* nospec r8 */
pushq $0 /* pt_regs->r9 = 0 */
xorq %r9, %r9 /* nospec r9 */
xorl %r9d, %r9d /* nospec r9 */
pushq $0 /* pt_regs->r10 = 0 */
xorq %r10, %r10 /* nospec r10 */
xorl %r10d, %r10d /* nospec r10 */
pushq $0 /* pt_regs->r11 = 0 */
xorq %r11, %r11 /* nospec r11 */
xorl %r11d, %r11d /* nospec r11 */
pushq %rbx /* pt_regs->rbx */
xorl %ebx, %ebx /* nospec rbx */
pushq %rbp /* pt_regs->rbp */
xorl %ebp, %ebp /* nospec rbp */
pushq %r12 /* pt_regs->r12 */
xorq %r12, %r12 /* nospec r12 */
xorl %r12d, %r12d /* nospec r12 */
pushq %r13 /* pt_regs->r13 */
xorq %r13, %r13 /* nospec r13 */
xorl %r13d, %r13d /* nospec r13 */
pushq %r14 /* pt_regs->r14 */
xorq %r14, %r14 /* nospec r14 */
xorl %r14d, %r14d /* nospec r14 */
pushq %r15 /* pt_regs->r15 */
xorq %r15, %r15 /* nospec r15 */
xorl %r15d, %r15d /* nospec r15 */
cld

/*
Expand Down

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