+Joerg
On 11/28/23 11:40, Tony Luck wrote:
On Tue, Nov 28, 2023 at 04:12:24PM +0300, Dan Carpenter wrote:
Hello Tony Luck,
The patch b041b525dab9: "x86/split_lock: Make life miserable for
split lockers" from Mar 10, 2022 (linux-next), leads to the following
Smatch static checker warning:
arch/x86/kernel/cpu/intel.c:1179 split_lock_warn()
warn: sleeping in atomic context
arch/x86/kernel/cpu/intel.c
1158 static void split_lock_warn(unsigned long ip)
1159 {
1160 struct delayed_work *work;
1161 int cpu;
1162
1163 if (!current->reported_split_lock)
1164 pr_warn_ratelimited("#AC: %s/%d took a split_lock trap at address: 0x%lx\n",
1165 current->comm, current->pid, ip);
1166 current->reported_split_lock = 1;
1167
1168 if (sysctl_sld_mitigate) {
1169 /*
1170 * misery factor #1:
1171 * sleep 10ms before trying to execute split lock.
1172 */
1173 if (msleep_interruptible(10) > 0)
1174 return;
1175 /*
1176 * Misery factor #2:
1177 * only allow one buslocked disabled core at a time.
1178 */
--> 1179 if (down_interruptible(&buslock_sem) == -EINTR)
1180 return;
1181 work = &sl_reenable_unlock;
1182 } else {
1183 work = &sl_reenable;
1184 }
1185
1186 cpu = get_cpu();
1187 schedule_delayed_work_on(cpu, work, 2);
1188
1189 /* Disable split lock detection on this CPU to make progress */
1190 sld_update_msr(false);
1191 put_cpu();
1192 }
The call tree is:
kernel_exc_vmm_communication() <- disables preempt
-> vc_raw_handle_exception()
-> vc_forward_exception()
-> exc_alignment_check()
-> __exc_alignment_check()
-> handle_user_split_lock()
-> split_lock_warn()
I think maybe the mismatch is that kernel_exc_vmm_communication() calls
irqentry_nmi_enter(regs); which disable preemption but exc_alignment_check()
does local_irq_enable() which doesn't enable it.
I think we need some arch/x86/kernel/sev.c expertise to explain the
preemption requirements in that stack trace. Adding Tom Lendacky.
Adding Joerg as the original developer of this code.
I believe that irqentry_nmi_enter() is used so that we are ensured that
the kernel can't be interrupted while using the per-CPU GHCB when entered
from kernel-mode in order to avoid nested #VCs (except for an NMI). Joerg
might have further insights since there was a lot of discussion around
these changes.
I'm not sure if is possible, but I wonder if irqentry_nmi_exit() can be
issued before forwarding the exception - or even delay forwarding the
exception until after irqentry_nmi_exit().
Thanks,
Tom
Also why does arch/x86 not have a dedicated mailing list?
Good question. X86 was once the default architecture. So everything went to
linux-kernel@xxxxxxxxxxxxxxx. I'll add that to Cc: for this. But maybe
it's time for an x86 specific list?
regards,
dan carpenter
-Tony
