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a493d1ca1a
sync_core_before_usermode() had an incorrect optimization. If the kernel
returns from an interrupt, it can get to usermode without IRET. It just has
to schedule to a different task in the same mm and do SYSRET. Fortunately,
there were no callers of sync_core_before_usermode() that could have had
in_irq() or in_nmi() equal to true, because it's only ever called from the
scheduler.
While at it, clarify a related comment.
Fixes: 70216e18e5 ("membarrier: Provide core serializing command, *_SYNC_CORE")
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/5afc7632be1422f91eaf7611aaaa1b5b8580a086.1607058304.git.luto@kernel.org
112 lines
3.2 KiB
C
112 lines
3.2 KiB
C
/* SPDX-License-Identifier: GPL-2.0 */
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#ifndef _ASM_X86_SYNC_CORE_H
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#define _ASM_X86_SYNC_CORE_H
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#include <linux/preempt.h>
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#include <asm/processor.h>
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#include <asm/cpufeature.h>
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#include <asm/special_insns.h>
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#ifdef CONFIG_X86_32
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static inline void iret_to_self(void)
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{
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asm volatile (
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"pushfl\n\t"
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"pushl %%cs\n\t"
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"pushl $1f\n\t"
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"iret\n\t"
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"1:"
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: ASM_CALL_CONSTRAINT : : "memory");
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}
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#else
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static inline void iret_to_self(void)
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{
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unsigned int tmp;
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asm volatile (
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"mov %%ss, %0\n\t"
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"pushq %q0\n\t"
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"pushq %%rsp\n\t"
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"addq $8, (%%rsp)\n\t"
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"pushfq\n\t"
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"mov %%cs, %0\n\t"
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"pushq %q0\n\t"
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"pushq $1f\n\t"
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"iretq\n\t"
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"1:"
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: "=&r" (tmp), ASM_CALL_CONSTRAINT : : "cc", "memory");
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}
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#endif /* CONFIG_X86_32 */
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/*
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* This function forces the icache and prefetched instruction stream to
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* catch up with reality in two very specific cases:
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*
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* a) Text was modified using one virtual address and is about to be executed
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* from the same physical page at a different virtual address.
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*
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* b) Text was modified on a different CPU, may subsequently be
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* executed on this CPU, and you want to make sure the new version
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* gets executed. This generally means you're calling this in an IPI.
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*
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* If you're calling this for a different reason, you're probably doing
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* it wrong.
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*
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* Like all of Linux's memory ordering operations, this is a
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* compiler barrier as well.
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*/
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static inline void sync_core(void)
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{
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/*
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* The SERIALIZE instruction is the most straightforward way to
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* do this, but it is not universally available.
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*/
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if (static_cpu_has(X86_FEATURE_SERIALIZE)) {
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serialize();
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return;
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}
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/*
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* For all other processors, there are quite a few ways to do this.
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* IRET-to-self is nice because it works on every CPU, at any CPL
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* (so it's compatible with paravirtualization), and it never exits
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* to a hypervisor. The only downsides are that it's a bit slow
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* (it seems to be a bit more than 2x slower than the fastest
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* options) and that it unmasks NMIs. The "push %cs" is needed,
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* because in paravirtual environments __KERNEL_CS may not be a
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* valid CS value when we do IRET directly.
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*
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* In case NMI unmasking or performance ever becomes a problem,
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* the next best option appears to be MOV-to-CR2 and an
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* unconditional jump. That sequence also works on all CPUs,
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* but it will fault at CPL3 (i.e. Xen PV).
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*
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* CPUID is the conventional way, but it's nasty: it doesn't
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* exist on some 486-like CPUs, and it usually exits to a
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* hypervisor.
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*/
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iret_to_self();
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}
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/*
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* Ensure that a core serializing instruction is issued before returning
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* to user-mode. x86 implements return to user-space through sysexit,
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* sysrel, and sysretq, which are not core serializing.
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*/
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static inline void sync_core_before_usermode(void)
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{
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/* With PTI, we unconditionally serialize before running user code. */
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if (static_cpu_has(X86_FEATURE_PTI))
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return;
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/*
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* Even if we're in an interrupt, we might reschedule before returning,
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* in which case we could switch to a different thread in the same mm
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* and return using SYSRET or SYSEXIT. Instead of trying to keep
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* track of our need to sync the core, just sync right away.
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*/
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sync_core();
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}
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#endif /* _ASM_X86_SYNC_CORE_H */
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