Re: [patch 4/8] Immediate Value - i386 Optimization

Mathieu Desnoyers
2007-06-17T18:19:52Z

* Chuck Ebbert (cebbert@redhat.com) wrote:
> On 06/15/2007 04:23 PM, Mathieu Desnoyers wrote:
> > i386 optimization of the immediate values which uses a movl with code patching
> > to set/unset the value used to populate the register used for the branch test.
> >
> > Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
> > ---
> > arch/i386/kernel/Makefile | 1
> > arch/i386/kernel/immediate.c | 177 +++++++++++++++++++++++++++++++++++++++++++
> > include/asm-i386/immediate.h | 72 +++++++++++++++++
> > 3 files changed, 249 insertions(+), 1 deletion(-)
> >
> > Index: linux-2.6-lttng/include/asm-i386/immediate.h
> > ===================================================================
> > --- linux-2.6-lttng.orig/include/asm-i386/immediate.h 2007-06-15 16:13:55.000000000 -0400
> > +++ linux-2.6-lttng/include/asm-i386/immediate.h 2007-06-15 16:14:04.000000000 -0400
> > @@ -1 +1,71 @@
> > -#include <asm-generic/immediate.h>
> > +#ifndef _ASM_I386_IMMEDIATE_H
> > +#define _ASM_I386_IMMEDIATE_H
> > +
> > +/*
> > + * Immediate values. i386 architecture optimizations.
> > + *
> > + * (C) Copyright 2006 Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
> > + *
> > + * This file is released under the GPLv2.
> > + * See the file COPYING for more details.
> > + */
> > +
> > +#define IF_DEFAULT (IF_OPTIMIZED | IF_LOCKDEP)
> > +
> > +/*
> > + * Optimized version of the immediate. Passing the flags as a pointer to
> > + * the inline assembly to trick it into putting the flags value as third
> > + * parameter in the structure.
> > + */
> > +#define immediate_optimized(flags, var) \
> > + ({ \
> > + int condition; \
> > + asm ( ".section __immediate, \"a\", @progbits;\n\t" \
> > + ".long %1, 0f, %2;\n\t" \
> > + ".previous;\n\t" \
> > + "0:\n\t" \
> > + "movl %3,%0;\n\t" \
> > + : "=r" (condition) \
> > + : "m" (var), \
> > + "m" (*(char*)flags), \
> > + "i" (0)); \
> > + (condition); \
>
Hi Chuck,

> Unnecessary parens.
>

ok, fixed in each immediate.h.

> > + })
> > +
> > +/*
> > + * immediate macro selecting the generic or optimized version of immediate,
> > + * depending on the flags specified. It is a macro because we need to pass the
> > + * name to immediate_optimized() and immediate_generic() so they can declare a
> > + * static variable with it.
> > + */
> > +#define _immediate(flags, var) \
> > +({ \
> > + (((flags) & IF_LOCKDEP) && ((flags) & IF_OPTIMIZED)) ? \
> > + immediate_optimized(flags, var) : \
> > + immediate_generic(flags, var); \
> > +})
> > +
> > +/* immediate with default behavior */
> > +#define immediate(var) _immediate(IF_DEFAULT, var)
> > +
> > +/*
> > + * Architecture dependant immediate information, used internally for immediate
> > + * activation.
> > + */
> > +
> > +/*
> > + * Offset of the immediate value from the start of the movl instruction, in
> > + * bytes. We point to the first lower byte of the 4 bytes immediate value. Only
> > + * changing one byte makes sure we do an atomic memory write, independently of
> > + * the alignment of the 4 bytes in the load immediate instruction.
> > + */
> > +#define IMMEDIATE_OPTIMIZED_ENABLE_IMMEDIATE_OFFSET 1
> > +#define IMMEDIATE_OPTIMIZED_ENABLE_TYPE unsigned char
> > +/* Dereference enable as lvalue from a pointer to its instruction */
> > +#define IMMEDIATE_OPTIMIZED_ENABLE(a) \
> > + (*(IMMEDIATE_OPTIMIZED_ENABLE_TYPE*) \
> > + ((char*)(a)+IMMEDIATE_OPTIMIZED_ENABLE_IMMEDIATE_OFFSET))
> > +
> > +extern int immediate_optimized_set_enable(void *address, char enable);
> > +
> > +#endif /* _ASM_I386_IMMEDIATE_H */
> > Index: linux-2.6-lttng/arch/i386/kernel/Makefile
> > ===================================================================
> > --- linux-2.6-lttng.orig/arch/i386/kernel/Makefile 2007-06-15 16:13:51.000000000 -0400
> > +++ linux-2.6-lttng/arch/i386/kernel/Makefile 2007-06-15 16:14:04.000000000 -0400
> > @@ -35,6 +35,7 @@
> > obj-y += sysenter.o vsyscall.o
> > obj-$(CONFIG_ACPI_SRAT) += srat.o
> > obj-$(CONFIG_EFI) += efi.o efi_stub.o
> > +obj-$(CONFIG_IMMEDIATE) += immediate.o
> > obj-$(CONFIG_DOUBLEFAULT) += doublefault.o
> > obj-$(CONFIG_SERIAL_8250) += legacy_serial.o
> > obj-$(CONFIG_VM86) += vm86.o
> > Index: linux-2.6-lttng/arch/i386/kernel/immediate.c
> > ===================================================================
> > --- /dev/null 1970-01-01 00:00:00.000000000 +0000
> > +++ linux-2.6-lttng/arch/i386/kernel/immediate.c 2007-06-15 16:14:04.000000000 -0400
> > @@ -0,0 +1,177 @@
> > +/*
> > + * Immediate Value - i386 architecture specific code.
> > + *
> > + * Rationale
> > + *
> > + * Required because of :
> > + * - Erratum 49 fix for Intel PIII.
> > + * - Still present on newer processors : Intel Core 2 Duo Processor for Intel
> > + * Centrino Duo Processor Technology Specification Update, AH33.
> > + * Unsynchronized Cross-Modifying Code Operations Can Cause Unexpected
> > + * Instruction Execution Results.
> > + *
> > + * Permits immediate value modification by XMC with correct serialization.
> > + *
> > + * Reentrant for NMI and trap handler instrumentation. Permits XMC to a
> > + * location that has preemption enabled because it involves no temporary or
> > + * reused data structure.
> > + *
> > + * Quoting Richard J Moore, source of the information motivating this
> > + * implementation which differs from the one proposed by Intel which is not
> > + * suitable for kernel context (does not support NMI and would require disabling
> > + * interrupts on every CPU for a long period) :
> > + *
> > + * "There is another issue to consider when looking into using probes other
> > + * then int3:
> > + *
> > + * Intel erratum 54 - Unsynchronized Cross-modifying code - refers to the
> > + * practice of modifying code on one processor where another has prefetched
> > + * the unmodified version of the code. Intel states that unpredictable general
> > + * protection faults may result if a synchronizing instruction (iret, int,
> > + * int3, cpuid, etc ) is not executed on the second processor before it
> > + * executes the pre-fetched out-of-date copy of the instruction.
> > + *
> > + * When we became aware of this I had a long discussion with Intel's
> > + * microarchitecture guys. It turns out that the reason for this erratum
> > + * (which incidentally Intel does not intend to fix) is because the trace
> > + * cache - the stream of micorops resulting from instruction interpretation -
> > + * cannot guaranteed to be valid. Reading between the lines I assume this
> > + * issue arises because of optimization done in the trace cache, where it is
> > + * no longer possible to identify the original instruction boundaries. If the
> > + * CPU discoverers that the trace cache has been invalidated because of
> > + * unsynchronized cross-modification then instruction execution will be
> > + * aborted with a GPF. Further discussion with Intel revealed that replacing
> > + * the first opcode byte with an int3 would not be subject to this erratum.
> > + *
> > + * So, is cmpxchg reliable? One has to guarantee more than mere atomicity."
> > + *
> > + * Overall design
> > + *
> > + * The algorithm proposed by Intel applies not so well in kernel context: it
> > + * would imply disabling interrupts and looping on every CPUs while modifying
> > + * the code and would not support instrumentation of code called from interrupt
> > + * sources that cannot be disabled.
> > + *
> > + * Therefore, we use a different algorithm to respect Intel's erratum (see the
> > + * quoted discussion above). We make sure that no CPU sees an out-of-date copy
> > + * of a pre-fetched instruction by 1 - using a breakpoint, which skips the
> > + * instruction that is going to be modified, 2 - issuing an IPI to every CPU to
> > + * execute a sync_core(), to make sure that even when the breakpoint is removed,
> > + * no cpu could possibly still have the out-of-date copy of the instruction,
> > + * modify the now unused 2nd byte of the instruction, and then put back the
> > + * original 1st byte of the instruction.
> > + *
> > + * It has exactly the same intent as the algorithm proposed by Intel, but
> > + * it has less side-effects, scales better and supports NMI, SMI and MCE.
>
> Algorithm looks plausible, was it really tested?
>

Yes, but I can't reproduce the erroneous condition on my setup. I just
realized that I did not test it after my last changes. Therefore, see
the following comments inline for the upcoming fixes :

> > + *
> > + * Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
> > + */
> > +
> > +#include <linux/notifier.h>
> > +#include <linux/preempt.h>
> > +#include <linux/smp.h>
> > +#include <linux/notifier.h>
> > +#include <linux/module.h>
> > +#include <linux/immediate.h>
> > +#include <linux/kdebug.h>
> > +
> > +#include <asm/cacheflush.h>
> > +
> > +#define BREAKPOINT_INSTRUCTION 0xcc
> > +#define BREAKPOINT_INS_LEN 1
> > +
> > +static long target_eip;
> > +
> > +static void immediate_synchronize_core(void *info)
> > +{
> > + sync_core(); /* use cpuid to stop speculative execution */
> > +}
> > +
> > +/*
> > + * The eip value points right after the breakpoint instruction, in the second
> > + * byte of the movb. Incrementing it of 1 byte makes the code resume right after
> > + * the movb instruction, effectively skipping this instruction.
> > + *
> > + * We simply skip the 2 bytes load immediate here, leaving the register in an
> > + * undefined state. We don't care about the content (0 or !0), because we are
> > + * changing the value 0->1 or 1->0. This small window of undefined value
> > + * doesn't matter.
> > + */
> > +static int immediate_notifier(struct notifier_block *nb,
> > + unsigned long val, void *data)
> > +{
> > + enum die_val die_val = (enum die_val) val;
> > + struct die_args *args = data;
> > +
> > + if (!args->regs || user_mode_vm(args->regs))
> > + return NOTIFY_DONE;
> > +
> > + if (die_val == DIE_INT3 && args->regs->eip == target_eip) {
> > + args->regs->eip += 1; /* Skip the next byte of load immediate */
>
> <nitpick>
> args->regs->eip++;
>

Should now be args->regs->eip += 4; because I jump over the last 4 bytes
of the 5 bytes long movl instead of jumping over the last byte of the 2 bytes
movb I used previously.

I also have to use the int3 handler installed by CONFIG_KPROBES when
CONFIG_IMMEDIATE is used so I do not depend on KPROBES for i386. It will
also be fixed in the next release.

> > + return NOTIFY_STOP;
> > + }
> > + return NOTIFY_DONE;
> > +}
> > +
> > +static struct notifier_block immediate_notify = {
> > + .notifier_call = immediate_notifier,
> > + .priority = 0x7fffffff, /* we need to be notified first */
> > +};
> > +
> > +/*
> > + * The address is not aligned. We can only change 1 byte of the value
> > + * atomically.
> > + * Must be called with immediate_mutex held.
> > + */
> > +int immediate_optimized_set_enable(void *address, char enable)
> > +{
> > + char saved_byte;
> > + int ret;
> > + char *dest = address;
> > +
> > + if (!(enable ^ dest[1])) /* Must be a state change 0<->1 to execute */
> > + return 0;
> > +
> > +#if defined(CONFIG_DEBUG_RODATA) || defined(CONFIG_DEBUG_PAGEALLOC)
> > + /* Make sure this page is writable */
> > + change_page_attr(virt_to_page(address), 1, PAGE_KERNEL_EXEC);
> > + global_flush_tlb();
> > +#endif
>
> Can't we have a macro or inline to do this, and the setting back
> to read-only? kprobes also has the same ugly #if blocks...
>
I guess it would be better to share a common macro between kprobes and
immediate for this.

> Hmm, what happens if you race with kprobes setting a probe on
> the same page? Couldn't it unexpectedly become read-only?
>

Sure it can. Is there any spinlock already there that could be used by
different objects and would fit this purpose?

> > + target_eip = (long)address + BREAKPOINT_INS_LEN;
> > + /* register_die_notifier has memory barriers */
> > + register_die_notifier(&immediate_notify);
> > + saved_byte = *dest;
> > + *dest = BREAKPOINT_INSTRUCTION;
> > + wmb();
> > + /*
> > + * Execute serializing instruction on each CPU.
> > + * Acts as a memory barrier.
> > + */
> > + ret = on_each_cpu(immediate_synchronize_core, NULL, 1, 1);
> > + BUG_ON(ret != 0);
> > +
> > + dest[1] = enable;
> > + wmb();
> > + *dest = saved_byte;
> > + /*
> > + * Wait for all int3 handlers to end
> > + * (interrupts are disabled in int3).
> > + * This CPU is clearly not in a int3 handler,
> > + * because int3 handler is not preemptible and
> > + * there cannot be any more int3 handler called
> > + * for this site, because we placed the original
> > + * instruction back.
> > + * synchronize_sched has memory barriers.
> > + */
> > + synchronize_sched();
> > + unregister_die_notifier(&immediate_notify);
> > + /* unregister_die_notifier has memory barriers */
> > + target_eip = 0;
> > +#if defined(CONFIG_DEBUG_RODATA)
> > + /* Set this page back to RX */
> > + change_page_attr(virt_to_page(address), 1, PAGE_KERNEL_RX);
> > + global_flush_tlb();
> > +#endif
> > + return 0;
> > +}
> > +EXPORT_SYMBOL_GPL(immediate_optimized_set_enable);
> >
>