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kernel_optimize_test/arch/mips/include/asm/mmu_context.h
Paul Burton 432c6bacbd MIPS: Use per-mm page to execute branch delay slot instructions 
In some cases the kernel needs to execute an instruction from the delay
slot of an emulated branch instruction. These cases include:

  - Emulated floating point branch instructions (bc1[ft]l?) for systems
    which don't include an FPU, or upon which the kernel is run with the
    "nofpu" parameter.

  - MIPSr6 systems running binaries targeting older revisions of the
    architecture, which may include branch instructions whose encodings
    are no longer valid in MIPSr6.

Executing instructions from such delay slots is done by writing the
instruction to memory followed by a trap, as part of an "emuframe", and
executing it. This avoids the requirement of an emulator for the entire
MIPS instruction set. Prior to this patch such emuframes are written to
the user stack and executed from there.

This patch moves FP branch delay emuframes off of the user stack and
into a per-mm page. Allocating a page per-mm leaves userland with access
to only what it had access to previously, and compared to other
solutions is relatively simple.

When a thread requires a delay slot emulation, it is allocated a frame.
A thread may only have one frame allocated at any one time, since it may
only ever be executing one instruction at any one time. In order to
ensure that we can free up allocated frame later, its index is recorded
in struct thread_struct. In the typical case, after executing the delay
slot instruction we'll execute a break instruction with the BRK_MEMU
code. This traps back to the kernel & leads to a call to do_dsemulret
which frees the allocated frame & moves the user PC back to the
instruction that would have executed following the emulated branch.
In some cases the delay slot instruction may be invalid, such as a
branch, or may trigger an exception. In these cases the BRK_MEMU break
instruction will not be hit. In order to ensure that frames are freed
this patch introduces dsemul_thread_cleanup() and calls it to free any
allocated frame upon thread exit. If the instruction generated an
exception & leads to a signal being delivered to the thread, or indeed
if a signal simply happens to be delivered to the thread whilst it is
executing from the struct emuframe, then we need to take care to exit
the frame appropriately. This is done by either rolling back the user PC
to the branch or advancing it to the continuation PC prior to signal
delivery, using dsemul_thread_rollback(). If this were not done then a
sigreturn would return to the struct emuframe, and if that frame had
meanwhile been used in response to an emulated branch instruction within
the signal handler then we would execute the wrong user code.

Whilst a user could theoretically place something like a compact branch
to self in a delay slot and cause their thread to become stuck in an
infinite loop with the frame never being deallocated, this would:

  - Only affect the users single process.

  - Be architecturally invalid since there would be a branch in the
    delay slot, which is forbidden.

  - Be extremely unlikely to happen by mistake, and provide a program
    with no more ability to harm the system than a simple infinite loop
    would.

If a thread requires a delay slot emulation & no frame is available to
it (ie. the process has enough other threads that all frames are
currently in use) then the thread joins a waitqueue. It will sleep until
a frame is freed by another thread in the process.

Since we now know whether a thread has an allocated frame due to our
tracking of its index, the cookie field of struct emuframe is removed as
we can be more certain whether we have a valid frame. Since a thread may
only ever have a single frame at any given time, the epc field of struct
emuframe is also removed & the PC to continue from is instead stored in
struct thread_struct. Together these changes simplify & shrink struct
emuframe somewhat, allowing twice as many frames to fit into the page
allocated for them.

The primary benefit of this patch is that we are now free to mark the
user stack non-executable where that is possible.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: Leonid Yegoshin <leonid.yegoshin@imgtec.com>
Cc: Maciej Rozycki <maciej.rozycki@imgtec.com>
Cc: Faraz Shahbazker <faraz.shahbazker@imgtec.com>
Cc: Raghu Gandham <raghu.gandham@imgtec.com>
Cc: Matthew Fortune <matthew.fortune@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/13764/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
2016-08-02 09:28:53 +02:00

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/*
* Switch a MMU context.
*
* This file is subject to the terms and conditions of the GNU General Public
* License. See the file "COPYING" in the main directory of this archive
* for more details.
*
* Copyright (C) 1996, 1997, 1998, 1999 by Ralf Baechle
* Copyright (C) 1999 Silicon Graphics, Inc.
*/
#ifndef _ASM_MMU_CONTEXT_H
#define _ASM_MMU_CONTEXT_H
#include <linux/errno.h>
#include <linux/sched.h>
#include <linux/smp.h>
#include <linux/slab.h>
#include <asm/cacheflush.h>
#include <asm/dsemul.h>
#include <asm/hazards.h>
#include <asm/tlbflush.h>
#include <asm-generic/mm_hooks.h>
#define htw_set_pwbase(pgd) \
do { \
if (cpu_has_htw) { \
write_c0_pwbase(pgd); \
back_to_back_c0_hazard(); \
} \
} while (0)
#define TLBMISS_HANDLER_SETUP_PGD(pgd) \
do { \
extern void tlbmiss_handler_setup_pgd(unsigned long); \
tlbmiss_handler_setup_pgd((unsigned long)(pgd)); \
htw_set_pwbase((unsigned long)pgd); \
} while (0)
#ifdef CONFIG_MIPS_PGD_C0_CONTEXT
#define TLBMISS_HANDLER_RESTORE() \
write_c0_xcontext((unsigned long) smp_processor_id() << \
SMP_CPUID_REGSHIFT)
#define TLBMISS_HANDLER_SETUP() \
do { \
TLBMISS_HANDLER_SETUP_PGD(swapper_pg_dir); \
TLBMISS_HANDLER_RESTORE(); \
} while (0)
#else /* !CONFIG_MIPS_PGD_C0_CONTEXT: using pgd_current*/
/*
* For the fast tlb miss handlers, we keep a per cpu array of pointers
* to the current pgd for each processor. Also, the proc. id is stuffed
* into the context register.
*/
extern unsigned long pgd_current[];
#define TLBMISS_HANDLER_RESTORE() \
write_c0_context((unsigned long) smp_processor_id() << \
SMP_CPUID_REGSHIFT)
#define TLBMISS_HANDLER_SETUP() \
TLBMISS_HANDLER_RESTORE(); \
back_to_back_c0_hazard(); \
TLBMISS_HANDLER_SETUP_PGD(swapper_pg_dir)
#endif /* CONFIG_MIPS_PGD_C0_CONTEXT*/
/*
* All unused by hardware upper bits will be considered
* as a software asid extension.
*/
static unsigned long asid_version_mask(unsigned int cpu)
{
unsigned long asid_mask = cpu_asid_mask(&cpu_data[cpu]);
return ~(asid_mask | (asid_mask - 1));
}
static unsigned long asid_first_version(unsigned int cpu)
{
return ~asid_version_mask(cpu) + 1;
}
#define cpu_context(cpu, mm) ((mm)->context.asid[cpu])
#define asid_cache(cpu) (cpu_data[cpu].asid_cache)
#define cpu_asid(cpu, mm) \
(cpu_context((cpu), (mm)) & cpu_asid_mask(&cpu_data[cpu]))
static inline void enter_lazy_tlb(struct mm_struct *mm, struct task_struct *tsk)
{
}
/* Normal, classic MIPS get_new_mmu_context */
static inline void
get_new_mmu_context(struct mm_struct *mm, unsigned long cpu)
{
extern void kvm_local_flush_tlb_all(void);
unsigned long asid = asid_cache(cpu);
if (!((asid += cpu_asid_inc()) & cpu_asid_mask(&cpu_data[cpu]))) {
if (cpu_has_vtag_icache)
flush_icache_all();
#ifdef CONFIG_KVM
kvm_local_flush_tlb_all(); /* start new asid cycle */
#else
local_flush_tlb_all(); /* start new asid cycle */
#endif
if (!asid) /* fix version if needed */
asid = asid_first_version(cpu);
}
cpu_context(cpu, mm) = asid_cache(cpu) = asid;
}
/*
* Initialize the context related info for a new mm_struct
* instance.
*/
static inline int
init_new_context(struct task_struct *tsk, struct mm_struct *mm)
{
int i;
for_each_possible_cpu(i)
cpu_context(i, mm) = 0;
atomic_set(&mm->context.fp_mode_switching, 0);
mm->context.bd_emupage_allocmap = NULL;
spin_lock_init(&mm->context.bd_emupage_lock);
init_waitqueue_head(&mm->context.bd_emupage_queue);
return 0;
}
static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next,
struct task_struct *tsk)
{
unsigned int cpu = smp_processor_id();
unsigned long flags;
local_irq_save(flags);
htw_stop();
/* Check if our ASID is of an older version and thus invalid */
if ((cpu_context(cpu, next) ^ asid_cache(cpu)) & asid_version_mask(cpu))
get_new_mmu_context(next, cpu);
write_c0_entryhi(cpu_asid(cpu, next));
TLBMISS_HANDLER_SETUP_PGD(next->pgd);
/*
* Mark current->active_mm as not "active" anymore.
* We don't want to mislead possible IPI tlb flush routines.
*/
cpumask_clear_cpu(cpu, mm_cpumask(prev));
cpumask_set_cpu(cpu, mm_cpumask(next));
htw_start();
local_irq_restore(flags);
}
/*
* Destroy context related info for an mm_struct that is about
* to be put to rest.
*/
static inline void destroy_context(struct mm_struct *mm)
{
dsemul_mm_cleanup(mm);
}
#define deactivate_mm(tsk, mm) do { } while (0)
/*
* After we have set current->mm to a new value, this activates
* the context for the new mm so we see the new mappings.
*/
static inline void
activate_mm(struct mm_struct *prev, struct mm_struct *next)
{
unsigned long flags;
unsigned int cpu = smp_processor_id();
local_irq_save(flags);
htw_stop();
/* Unconditionally get a new ASID. */
get_new_mmu_context(next, cpu);
write_c0_entryhi(cpu_asid(cpu, next));
TLBMISS_HANDLER_SETUP_PGD(next->pgd);
/* mark mmu ownership change */
cpumask_clear_cpu(cpu, mm_cpumask(prev));
cpumask_set_cpu(cpu, mm_cpumask(next));
htw_start();
local_irq_restore(flags);
}
/*
* If mm is currently active_mm, we can't really drop it. Instead,
* we will get a new one for it.
*/
static inline void
drop_mmu_context(struct mm_struct *mm, unsigned cpu)
{
unsigned long flags;
local_irq_save(flags);
htw_stop();
if (cpumask_test_cpu(cpu, mm_cpumask(mm))) {
get_new_mmu_context(mm, cpu);
write_c0_entryhi(cpu_asid(cpu, mm));
} else {
/* will get a new context next time */
cpu_context(cpu, mm) = 0;
}
htw_start();
local_irq_restore(flags);
}
#endif /* _ASM_MMU_CONTEXT_H */
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