Donate to e Foundation | Murena handsets with /e/OS | Own a part of Murena! Learn more

Commit 07d2a628 authored by Nicholas Piggin's avatar Nicholas Piggin Committed by Michael Ellerman
Browse files

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



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: default avatarNicholas Piggin <npiggin@gmail.com>
Signed-off-by: default avatarMichael Ellerman <mpe@ellerman.id.au>
parent 9145effd

arch/powerpc/include/asm/ppc-opcode.h

+4 −4
Original line number Diff line number Diff line
@@ -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 | \

arch/powerpc/kernel/entry_64.S

+0 −9
Original line number Diff line number Diff line
@@ -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

arch/powerpc/kernel/process.c

+26 −1
Original line number Diff line number Diff line
@@ -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;