If hw supports it, on atomic operation that requires exclusive ownership of more than one cache line, #AC is generated.
Thread can control it with sysarch(I386_SET_SPLITLOCK). The global default is set with hw.splitlock_force.
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Test program I used with simics:
/* $Id: splitlock.c,v 1.5 2026/09/18 16:39:53 kostik Exp kostik $ */
#include <sys/param.h>
#include <sys/mman.h>
#include <machine/atomic.h>
#include <machine/sysarch.h>
#include <err.h>
#include <signal.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#ifndef I386_SET_SPLITLOCK
#define I386_SET_SPLITLOCK 14
#define I386_GET_SPLITLOCK 15
#endif
static void
sigbus_handler(int signo, siginfo_t *si, void *mc)
{
printf("sig%d si_code %d addr %p\n", signo, si->si_code, si->si_addr);
exit(0);
}
int
main(void)
{
char *m;
struct sigaction sa;
int error, val;
val = 1;
error = sysarch(I386_SET_SPLITLOCK, &val);
if (error == -1)
err(1, "I386_SET_SPLITLOCK");
error = sysarch(I386_GET_SPLITLOCK, &val);
if (error == -1)
err(1, "I386_GET_SPLITLOCK");
printf("setlock: %d\n", val);
memset(&sa, 0, sizeof(sa));
sa.sa_sigaction = sigbus_handler;
sa.sa_flags = SA_SIGINFO;
if (sigaction(SIGBUS, &sa, NULL) == -1)
err(1, "sigaction");
m = mmap(NULL, PAGE_SIZE, PROT_READ | PROT_WRITE,
MAP_PRIVATE | MAP_ANON, -1, 0);
if (m == MAP_FAILED)
err(1, "mmap");
m += 256 - 1;
atomic_fetchadd_long((unsigned long *)m, 1);
}Comment Actions
Fix splitlock control on exec for non-native ABIs.
Cache MSR_MEMORY_CTL, avoiding msr read on the context switch.
Use non-seializing immediate form for WRMSR when available.
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If I understand correctly, #AC isn't raised if a splitlock operation happens while in kernel mode. But, what about places that use casuword() and friends? They will behave the same as before even if splitlock detection is enabled.
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Isn't casuword() executed in kernel mode?
Hm, I also had an impression that #AC only happen when CPL is 3. But re-checking SDM 11.1.2.3 Features to Disable Bus Locks and description of the #AC exception, I started doubt it.
I think it costs nothing to add the recovery for #AC handler in kernel mode.
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Add kernel-mode #AC handler.
Copy md_td_flags from the right source.
Use atomic_load() in context switch code.
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I checked, simics does generates #AC in CPL 0, the current patch correctly returns EFAULT from umtx_op UMTX_OP_MUTEX_LOCK on mis-aligned struct umutex.
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I wonder if it should be a procctl feature instead of sysarch()? If your intent is to set ia32_splitlock_force = 1 by default, then it would be nice if proccontrol could be used to override application settings without having to modify the program or change the global default. Hmm, but I guess this wouldn't work if the policy is to reset the splitlock setting upon exec.
| sys/amd64/amd64/exec_machdep.c | ||
|---|---|---|
| 415 | What if !ia32_splitlock is true? disable_splitlock() asserts MPASS(ia32_splitlock). | |
| sys/amd64/amd64/pmap.c | ||
| 10256 | I think the oldtd != td check is redundant: if oldtd == td then the oldpmap == pmap case above should be taken. | |
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My first version of the patch controlled it with MD procctl. But I felt it was too 'tweaky' for such minor and probably not too popular feature. After all, we do not provide a procctl knob to enable %rflags.AC, and nobody asked for that.
If somebody wants to enable either alignment checks or disable splitlocks, this can be done with a trivial LD_PRELOADed dso. The static binaries are off (like Go-compiled binaries) but I do not feel guilty.
Comment Actions
In exec_splitlock(), do not call disable_splitlock() if hw does not support it.
Remove oldtd != td check in context switch.