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powerpc/64s: Avoid cpabort in context switch when possible

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The ISA v3.0B copy-paste facility only requires cpabort when switching
to a process that has foreign real addresses mapped (direct access to
accelerators), to clear a potential copy buffer filled by a previous
thread. There is no accelerator driver implemented yet, so cpabort can
be removed. It can be be re-added when a driver is implemented.

POWER9 DD1 requires the copy buffer to always be cleared on context
switch, but if accelerators are not in use, then an unpaired copy from
a dummy region is sufficient to clear data out of the copy buffer.

This increases context switch performance by about 5% on POWER9.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
This commit is contained in:
Nicholas Piggin 2017-06-09 01:36:09 +10:00 committed by Michael Ellerman
parent 9145effd62
commit 07d2a628bc
3 changed files with 30 additions and 14 deletions
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8
arch/powerpc/include/asm/ppc-opcode.h
View file

@ -189,8 +189,7 @@
/* sorted alphabetically */
#define PPC_INST_BHRBE 0x7c00025c
#define PPC_INST_CLRBHRB 0x7c00035c
#define PPC_INST_COPY 0x7c00060c
#define PPC_INST_COPY_FIRST 0x7c20060c
#define PPC_INST_COPY 0x7c20060c
#define PPC_INST_CP_ABORT 0x7c00068c
#define PPC_INST_DCBA 0x7c0005ec
#define PPC_INST_DCBA_MASK 0xfc0007fe
@ -223,8 +222,7 @@
#define PPC_INST_MSGSNDP 0x7c00011c
#define PPC_INST_MTTMR 0x7c0003dc
#define PPC_INST_NOP 0x60000000
#define PPC_INST_PASTE 0x7c00070c
#define PPC_INST_PASTE_LAST 0x7c20070d
#define PPC_INST_PASTE 0x7c20070d
#define PPC_INST_POPCNTB 0x7c0000f4
#define PPC_INST_POPCNTB_MASK 0xfc0007fe
#define PPC_INST_POPCNTD 0x7c0003f4
@ -392,6 +390,8 @@
/* Deal with instructions that older assemblers aren't aware of */
#define PPC_CP_ABORT stringify_in_c(.long PPC_INST_CP_ABORT)
#define PPC_COPY(a, b) stringify_in_c(.long PPC_INST_COPY | \
___PPC_RA(a) | ___PPC_RB(b))
#define PPC_DCBAL(a, b) stringify_in_c(.long PPC_INST_DCBAL | \
__PPC_RA(a) | __PPC_RB(b))
#define PPC_DCBZL(a, b) stringify_in_c(.long PPC_INST_DCBZL | \

9
arch/powerpc/kernel/entry_64.S
View file

@ -536,15 +536,6 @@ _GLOBAL(_switch)
* which contains larx/stcx, which will clear any reservation
* of the task being switched.
*/
BEGIN_FTR_SECTION
/*
* A cp_abort (copy paste abort) here ensures that when context switching, a
* copy from one process can't leak into the paste of another.
*/
PPC_CP_ABORT
END_FTR_SECTION_IFSET(CPU_FTR_ARCH_300)
#ifdef CONFIG_PPC_BOOK3S
/* Cancel all explict user streams as they will have no use after context
* switch and will stop the HW from creating streams itself

27
arch/powerpc/kernel/process.c
View file

@ -1137,6 +1137,11 @@ static inline void restore_sprs(struct thread_struct *old_thread,
#endif
}
#ifdef CONFIG_PPC_BOOK3S_64
#define CP_SIZE 128
static const u8 dummy_copy_buffer[CP_SIZE] __attribute__((aligned(CP_SIZE)));
#endif
struct task_struct *__switch_to(struct task_struct *prev,
struct task_struct *new)
{
@ -1226,8 +1231,28 @@ struct task_struct *__switch_to(struct task_struct *prev,
batch->active = 1;
}
if (current_thread_info()->task->thread.regs)
if (current_thread_info()->task->thread.regs) {
restore_math(current_thread_info()->task->thread.regs);
/*
* The copy-paste buffer can only store into foreign real
* addresses, so unprivileged processes can not see the
* data or use it in any way unless they have foreign real
* mappings. We don't have a VAS driver that allocates those
* yet, so no cpabort is required.
*/
if (cpu_has_feature(CPU_FTR_POWER9_DD1)) {
/*
* DD1 allows paste into normal system memory, so we
* do an unpaired copy here to clear the buffer and
* prevent a covert channel being set up.
*
* cpabort is not used because it is quite expensive.
*/
asm volatile(PPC_COPY(%0, %1)
: : "r"(dummy_copy_buffer), "r"(0));
}
}
#endif /* CONFIG_PPC_STD_MMU_64 */
return last;