Re: [RFC PATCH] irqchip/gic-v3: Do not enable irqs when handling spurious interrups

[Marc Zyngier]

On Sat, 17 Apr 2021 03:01:54 +0100,
He Ying <heying24@xxxxxxxxxx> wrote:
> 
> Hello Marc,
> 
> 
> 在 2021/4/16 22:15, Marc Zyngier 写道:
> > [+ Mark]
> > 
> > On Fri, 16 Apr 2021 07:22:17 +0100,
> > He Ying <heying24@xxxxxxxxxx> wrote:
> >> We found this problem in our kernel src tree:
> >> 
> >> [   14.816231] ------------[ cut here ]------------
> >> [   14.816231] kernel BUG at irq.c:99!
> >> [   14.816232] Internal error: Oops - BUG: 0 [#1] SMP
> >> [   14.816232] Process swapper/0 (pid: 0, stack limit = 0x(____ptrval____))
> >> [   14.816233] CPU: 0 PID: 0 Comm: swapper/0 Tainted: G           O      4.19.95-1.h1.AOS2.0.aarch64 #14
> >> [   14.816233] Hardware name: evb (DT)
> >> [   14.816234] pstate: 80400085 (Nzcv daIf +PAN -UAO)
> >> [   14.816234] pc : asm_nmi_enter+0x94/0x98
> >> [   14.816235] lr : asm_nmi_enter+0x18/0x98
> >> [   14.816235] sp : ffff000008003c50
> >> [   14.816235] pmr_save: 00000070
> >> [   14.816237] x29: ffff000008003c50 x28: ffff0000095f56c0
> >> [   14.816238] x27: 0000000000000000 x26: ffff000008004000
> >> [   14.816239] x25: 00000000015e0000 x24: ffff8008fb916000
> >> [   14.816240] x23: 0000000020400005 x22: ffff0000080817cc
> >> [   14.816241] x21: ffff000008003da0 x20: 0000000000000060
> >> [   14.816242] x19: 00000000000003ff x18: ffffffffffffffff
> >> [   14.816243] x17: 0000000000000008 x16: 003d090000000000
> >> [   14.816244] x15: ffff0000095ea6c8 x14: ffff8008fff5ab40
> >> [   14.816244] x13: ffff8008fff58b9d x12: 0000000000000000
> >> [   14.816245] x11: ffff000008c8a200 x10: 000000008e31fca5
> >> [   14.816246] x9 : ffff000008c8a208 x8 : 000000000000000f
> >> [   14.816247] x7 : 0000000000000004 x6 : ffff8008fff58b9e
> >> [   14.816248] x5 : 0000000000000000 x4 : 0000000080000000
> >> [   14.816249] x3 : 0000000000000000 x2 : 0000000080000000
> >> [   14.816250] x1 : 0000000000120000 x0 : ffff0000095f56c0
> >> [   14.816251] Call trace:
> >> [   14.816251]  asm_nmi_enter+0x94/0x98
> >> [   14.816251]  el1_irq+0x8c/0x180
> >> [   14.816252]  gic_handle_irq+0xbc/0x2e4
> >> [   14.816252]  el1_irq+0xcc/0x180
> >> [   14.816253]  arch_timer_handler_virt+0x38/0x58
> >> [   14.816253]  handle_percpu_devid_irq+0x90/0x240
> >> [   14.816253]  generic_handle_irq+0x34/0x50
> >> [   14.816254]  __handle_domain_irq+0x68/0xc0
> >> [   14.816254]  gic_handle_irq+0xf8/0x2e4
> >> [   14.816255]  el1_irq+0xcc/0x180
> >> [   14.816255]  arch_cpu_idle+0x34/0x1c8
> >> [   14.816255]  default_idle_call+0x24/0x44
> >> [   14.816256]  do_idle+0x1d0/0x2c8
> >> [   14.816256]  cpu_startup_entry+0x28/0x30
> >> [   14.816256]  rest_init+0xb8/0xc8
> >> [   14.816257]  start_kernel+0x4c8/0x4f4
> >> [   14.816257] Code: 940587f1 d5384100 b9401001 36a7fd01 (d4210000)
> >> [   14.816258] Modules linked in: start_dp(O) smeth(O)
> >> [   15.103092] ---[ end trace 701753956cb14aa8 ]---
> >> [   15.103093] Kernel panic - not syncing: Fatal exception in interrupt
> >> [   15.103099] SMP: stopping secondary CPUs
> >> [   15.103100] Kernel Offset: disabled
> >> [   15.103100] CPU features: 0x36,a2400218
> >> [   15.103100] Memory Limit: none
> > Urgh...
> > 
> >> Our kernel src tree is based on 4.19.95 and backports arm64 pseudo-NMI
> >> patches but doesn't support nested NMI. Its top relative commit is
> >> commit 17ce302f3117 ("arm64: Fix interrupt tracing in the presence of NMIs").
> > Can you please reproduce it with mainline and without any backport?
> > It is hard to reason about something that isn't a vanilla kernel.
> 
> I think our kernel is quite like v5.3 mainline. Reproducing it in
> v5.3 mainline may be a little difficult for us because our product
> needs some more self developed patches to work.

I don't really care about 5.3. What I care about is the tip of the
tree, and anything we fix there can trickle down to the previous
stable releases.

> >> I look into this issue and find that it's caused by 'BUG_ON(in_nmi())'
> >> in nmi_enter(). From the call trace, we find two 'el1_irqs' which
> >> means an interrupt preempts the other one and the new one is an NMI.
> >> Furthermore, by adding some prints, we find the first irq also calls
> >> nmi_enter(), but its priority is not GICD_INT_NMI_PRI and its irq number
> >> is 1023. It enables irq by calling gic_arch_enable_irqs() in
> >> gic_handle_irq(). At this moment, the second irq preempts the first irq
> >> and it's an NMI but current context is already in nmi. So that may be
> >> the problem.
> > I'm not sure I get it. From the stack trace, I see this:
> > 
> > [   14.816251]  asm_nmi_enter+0x94/0x98
> > [   14.816251]  el1_irq+0x8c/0x180			(C)
> > [   14.816252]  gic_handle_irq+0xbc/0x2e4
> > [   14.816252]  el1_irq+0xcc/0x180			(B)
> > [   14.816253]  arch_timer_handler_virt+0x38/0x58
> > [   14.816253]  handle_percpu_devid_irq+0x90/0x240
> > [   14.816253]  generic_handle_irq+0x34/0x50
> > [   14.816254]  __handle_domain_irq+0x68/0xc0
> > [   14.816254]  gic_handle_irq+0xf8/0x2e4
> > [   14.816255]  el1_irq+0xcc/0x180			(A)
> > 
> > which indicates that we preempted a timer interrupt (A) with another
> > IRQ (B), itself immediately preempted by another IRQ (C)? That's
> > indeed at least one too many.
> > 
> > Can you please describe for each of (A), (B) and (C) whether they are
> > spurious or not, what their priorities are if they aren't spurious?
> 
> Yes. I ignored interrupt (A). (B) is spurious and its priority is
> 0xa0 and PMR is 0x70. (C) is an NMI and its priority is 0x20. Note
> that GIC_PRIO_IRQON is 0xe0, GIC_PRIO_IRQOFF is 0x60,
> GICD_INT_DEF_PRI is 0xa0 and GICD_INT_NMI_PRI is 0x20 in our kernel.

If (B) is spurious (aka ICC_IAR1R_EL1 return 1023), then its
"priority" doesn't really exist, and I don't really get what you mean
by "its priority is 0xa0".  ICC_RPR_EL1 shouldn't change when Ack-ing
a spurious interrupt, because there is no change in GIC state at all.

And if PMR is 0x70 at the point where you get (B), then I really can't
see how you can get an interrupt of priority 0xa0 anyway.



> >> In my opinion, when handling spurious interrupts, we shouldn't enable irqs.
> >> My reason is that for spurious interrupts we may enter nmi context in
> >> el1_irq() because current PMR may be GIC_PRIO_IRQOFF. If we enable irqs
> >> at this time, another NMI may happen and preempt this spurious interrupt
> >> but the context is already in nmi. That causes a bug on if nested NMI is
> >> not supported. Even for nested nmi, I think it's not a normal scenario.
> > I would tend to agree that this isn't great. Actually, I'd probably
> > move the check for a spurious interrupt right after the read of
> > ICC_IAR1_EL1, because there is no real need to do anything else at
> > that point.
> 
> So, we don't need to check NMI for spurious interrupts? Do you mean
> that spurious interrupts' can't be NMIs? Or even spurious interrups
> are NMIs, we shouldn't do anything for them? If so, I will move the
> check after the read of ICC_IAR1_EL1 and send a V2.

Spurious interrupts are not interrupts at all. It is either a level
interrupt that has been retired before being handled, or some other
transient effect (like an interrupt being moved from one CPU to
another), or even flaky HW. So doing anything based on a spurious
interrupt is definitely a potential bug, and I suggest this:

diff --git a/drivers/irqchip/irq-gic-v3.c b/drivers/irqchip/irq-gic-v3.c
index eb0ee356a629..00404024d7cd 100644
--- a/drivers/irqchip/irq-gic-v3.c
+++ b/drivers/irqchip/irq-gic-v3.c
@@ -648,6 +648,10 @@ static asmlinkage void __exception_irq_entry gic_handle_irq(struct pt_regs *regs
 
 	irqnr = gic_read_iar();
 
+	/* Check for special IDs first */
+	if ((irqnr >= 1020 && irqnr <= 1023))
+		return;
+
 	if (gic_supports_nmi() &&
 	    unlikely(gic_read_rpr() == GICD_INT_NMI_PRI)) {
 		gic_handle_nmi(irqnr, regs);
@@ -659,10 +663,6 @@ static asmlinkage void __exception_irq_entry gic_handle_irq(struct pt_regs *regs
 		gic_arch_enable_irqs();
 	}
 
-	/* Check for special IDs first */
-	if ((irqnr >= 1020 && irqnr <= 1023))
-		return;
-
 	if (static_branch_likely(&supports_deactivate_key))
 		gic_write_eoir(irqnr);
 	else

[...]

> In my opinion, since commit 17ce302f3117 ("arm64: Fix interrupt
> tracing in the presence of NMIs"), spurious interrups can enter nmi
> context in interrupt entry because PMR can be GIC_PRIO_IRQOFF for
> spurious interrupts. That means test_irqs_unmasked is not 0 and
> asm_nmi_enter is called.
> 
>    (some el1_irq entry code from v5.3)
>    test_irqs_unmasked  res=x0, pmr=x20
>    cbz x0, 1f
>    bl asm_nmi_enter

That code has significantly changed upstream, and is now in C. I think
it still do the same thing though.

> 
> And it then calls gic_handle_irq(). It doesn't call gic_handle_nmi()
> because its priority is not GICD_INT_NMI_PRI.
> 
> Then it enables irqs. If at that point another NMI comes and
> preempts it, which means NMI occurs in nmi context. That may cause a
> bug on if nested NMI is not supported.
> 
>    (some gic_handle_irq code from v5.3)
> 
>    irqnr = gic_read_iar();
> 
>    if (gic_supports_nmi() &&
>        unlikely(gic_read_rpr() == GICD_INT_NMI_PRI)) {
>            gic_handle_nmi(irqnr,
> regs);                                   (C)
>            return;
>    }
> 
>    if (gic_prio_masking_enabled()) {
>          gic_pmr_mask_irqs();
> gic_arch_enable_irqs(); (D)
>    }

I believe the above patch would fix the spurious interrupt issue you
have experienced. Please let me know, and post a v2 if this works for
you.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.