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Commit e00b12e6 authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Ingo Molnar
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perf/x86: Further optimize copy_from_user_nmi() 



Now that we can deal with nested NMI due to IRET re-enabling NMIs and
can deal with faults from NMI by making sure we preserve CR2 over NMIs
we can in fact simply access user-space memory from NMI context.

So rewrite copy_from_user_nmi() to use __copy_from_user_inatomic() and
rework the fault path to do the minimal required work before taking
the in_atomic() fault handler.

In particular avoid perf_sw_event() which would make perf recurse on
itself (it should be harmless as our recursion protections should be
able to deal with this -- but why tempt fate).

Also rename notify_page_fault() to kprobes_fault() as that is a much
better name; there is no notifier in it and its specific to kprobes.

Don measured that his worst case NMI path shrunk from ~300K cycles to
~150K cycles.

Cc: Stephane Eranian <eranian@google.com>
Cc: jmario@redhat.com
Cc: Arnaldo Carvalho de Melo <acme@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: dave.hansen@linux.intel.com
Tested-by: default avatarDon Zickus <dzickus@redhat.com>
Signed-off-by: default avatarPeter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20131024105206.GM2490@laptop.programming.kicks-ass.net


Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent 2c42cfbf

arch/x86/lib/usercopy.c

+15 −28
Original line number Diff line number Diff line
@@ -11,39 +11,26 @@ 
#include <linux/sched.h>



/*

 * best effort, GUP based copy_from_user() that is NMI-safe

 * We rely on the nested NMI work to allow atomic faults from the NMI path; the

 * nested NMI paths are careful to preserve CR2.

 */

unsigned long

copy_from_user_nmi(void *to, const void __user *from, unsigned long n)

{

	unsigned long offset, addr = (unsigned long)from;

	unsigned long size, len = 0;

	struct page *page;

	void *map;

	int ret;

	unsigned long ret;



	if (__range_not_ok(from, n, TASK_SIZE))

		return len;

		return 0;



	do {

		ret = __get_user_pages_fast(addr, 1, 0, &page);

		if (!ret)

			break;



		offset = addr & (PAGE_SIZE - 1);

		size = min(PAGE_SIZE - offset, n - len);



		map = kmap_atomic(page);

		memcpy(to, map+offset, size);

		kunmap_atomic(map);

		put_page(page);



		len  += size;

		to   += size;

		addr += size;



	} while (len < n);

	/*

	 * Even though this function is typically called from NMI/IRQ context

	 * disable pagefaults so that its behaviour is consistent even when

	 * called form other contexts.

	 */

	pagefault_disable();

	ret = __copy_from_user_inatomic(to, from, n);

	pagefault_enable();



	return len;

	return n - ret;

}

EXPORT_SYMBOL_GPL(copy_from_user_nmi);

arch/x86/mm/fault.c

+21 −20
Original line number Diff line number Diff line
@@ -51,7 +51,7 @@ kmmio_fault(struct pt_regs *regs, unsigned long addr) 
	return 0;

}



static inline int __kprobes notify_page_fault(struct pt_regs *regs)

static inline int __kprobes kprobes_fault(struct pt_regs *regs)

{

	int ret = 0;
@@ -1048,7 +1048,7 @@ __do_page_fault(struct pt_regs *regs, unsigned long error_code) 
			return;



		/* kprobes don't want to hook the spurious faults: */

		if (notify_page_fault(regs))

		if (kprobes_fault(regs))

			return;

		/*

		 * Don't take the mm semaphore here. If we fixup a prefetch
@@ -1060,23 +1060,8 @@ __do_page_fault(struct pt_regs *regs, unsigned long error_code) 
	}



	/* kprobes don't want to hook the spurious faults: */

	if (unlikely(notify_page_fault(regs)))

	if (unlikely(kprobes_fault(regs)))

		return;

	/*

	 * It's safe to allow irq's after cr2 has been saved and the

	 * vmalloc fault has been handled.

	 *

	 * User-mode registers count as a user access even for any

	 * potential system fault or CPU buglet:

	 */

	if (user_mode_vm(regs)) {

		local_irq_enable();

		error_code |= PF_USER;

		flags |= FAULT_FLAG_USER;

	} else {

		if (regs->flags & X86_EFLAGS_IF)

			local_irq_enable();

	}



	if (unlikely(error_code & PF_RSVD))

		pgtable_bad(regs, error_code, address);
@@ -1088,8 +1073,6 @@ __do_page_fault(struct pt_regs *regs, unsigned long error_code) 
		}

	}



	perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, address);



	/*

	 * If we're in an interrupt, have no user context or are running

	 * in an atomic region then we must not take the fault:
@@ -1099,6 +1082,24 @@ __do_page_fault(struct pt_regs *regs, unsigned long error_code) 
		return;

	}



	/*

	 * It's safe to allow irq's after cr2 has been saved and the

	 * vmalloc fault has been handled.

	 *

	 * User-mode registers count as a user access even for any

	 * potential system fault or CPU buglet:

	 */

	if (user_mode_vm(regs)) {

		local_irq_enable();

		error_code |= PF_USER;

		flags |= FAULT_FLAG_USER;

	} else {

		if (regs->flags & X86_EFLAGS_IF)

			local_irq_enable();

	}



	perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, address);



	if (error_code & PF_WRITE)

		flags |= FAULT_FLAG_WRITE;