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SubjectRe: [RFC PATCH] KVM: arm/arm64: vgic: change condition for level interrupt resampling
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Hi Marc,

On 08/03/18 12:54, Marc Zyngier wrote:
> On 08/03/18 09:49, Marc Zyngier wrote:
>> [updated Christoffer's email address]
>>
>> Hi Shunyong,
>>
>> On 08/03/18 07:01, Shunyong Yang wrote:
>>> When resampling irqfds is enabled, level interrupt should be
>>> de-asserted when resampling happens. On page 4-47 of GIC v3
>>> specification IHI0069D, it said,
>>> "When the PE acknowledges an SGI, a PPI, or an SPI at the CPU
>>> interface, the IRI changes the status of the interrupt to active
>>> and pending if:
>>> • It is an edge-triggered interrupt, and another edge has been
>>> detected since the interrupt was acknowledged.
>>> • It is a level-sensitive interrupt, and the level has not been
>>> deasserted since the interrupt was acknowledged."
>>>
>>> GIC v2 specification IHI0048B.b has similar description on page
>>> 3-42 for state machine transition.
>>>
>>> When some VFIO device, like mtty(8250 VFIO mdev emulation driver
>>> in samples/vfio-mdev) triggers a level interrupt, the status
>>> transition in LR is pending-->active-->active and pending.
>>> Then it will wait resampling to de-assert the interrupt.
>>>
>>> Current design of lr_signals_eoi_mi() will return false if state
>>> in LR is not invalid(Inactive). It causes resampling will not happen
>>> in mtty case.
>>
>> Let me rephrase this, and tell me if I understood it correctly:
>>
>> - A level interrupt is injected, activated by the guest (LR state=active)
>> - guest exits, re-enters, (LR state=pending+active)
>> - guest EOIs the interrupt (LR state=pending)
>> - maintenance interrupt
>> - we don't signal the resampling because we're not in an invalid state
>>
>> Is that correct?
>>
>> That's an interesting case, because it seems to invalidate some of the
>> optimization that went in over a year ago.
>>
>> 096f31c4360f KVM: arm/arm64: vgic: Get rid of MISR and EISR fields
>> b6095b084d87 KVM: arm/arm64: vgic: Get rid of unnecessary save_maint_int_state
>> af0614991ab6 KVM: arm/arm64: vgic: Get rid of unnecessary process_maintenance operation
>>
>> We could compare the value of the LR before the guest entry with
>> the value at exit time, but we still could miss it if we have a
>> transition such as P+A -> P -> A and assume a long enough propagation
>> delay for the maintenance interrupt (which is very likely).
>>
>> In essence, we have lost the benefit of EISR, which was to give us a
>> way to deal with asynchronous signalling.
>>
>>>
>>> This will cause interrupt fired continuously to guest even 8250 IIR
>>> has no interrupt. When 8250's interrupt is configured in shared mode,
>>> it will pass interrupt to other drivers to handle. However, there
>>> is no other driver involved. Then, a "nobody cared" kernel complaint
>>> occurs.
>>>
>>> / # cat /dev/ttyS0
>>> [ 4.826836] random: crng init done
>>> [ 6.373620] irq 41: nobody cared (try booting with the "irqpoll"
>>> option)
>>> [ 6.376414] CPU: 0 PID: 1307 Comm: cat Not tainted 4.16.0-rc4 #4
>>> [ 6.378927] Hardware name: linux,dummy-virt (DT)
>>> [ 6.380876] Call trace:
>>> [ 6.381937] dump_backtrace+0x0/0x180
>>> [ 6.383495] show_stack+0x14/0x1c
>>> [ 6.384902] dump_stack+0x90/0xb4
>>> [ 6.386312] __report_bad_irq+0x38/0xe0
>>> [ 6.387944] note_interrupt+0x1f4/0x2b8
>>> [ 6.389568] handle_irq_event_percpu+0x54/0x7c
>>> [ 6.391433] handle_irq_event+0x44/0x74
>>> [ 6.393056] handle_fasteoi_irq+0x9c/0x154
>>> [ 6.394784] generic_handle_irq+0x24/0x38
>>> [ 6.396483] __handle_domain_irq+0x60/0xb4
>>> [ 6.398207] gic_handle_irq+0x98/0x1b0
>>> [ 6.399796] el1_irq+0xb0/0x128
>>> [ 6.401138] _raw_spin_unlock_irqrestore+0x18/0x40
>>> [ 6.403149] __setup_irq+0x41c/0x678
>>> [ 6.404669] request_threaded_irq+0xe0/0x190
>>> [ 6.406474] univ8250_setup_irq+0x208/0x234
>>> [ 6.408250] serial8250_do_startup+0x1b4/0x754
>>> [ 6.410123] serial8250_startup+0x20/0x28
>>> [ 6.411826] uart_startup.part.21+0x78/0x144
>>> [ 6.413633] uart_port_activate+0x50/0x68
>>> [ 6.415328] tty_port_open+0x84/0xd4
>>> [ 6.416851] uart_open+0x34/0x44
>>> [ 6.418229] tty_open+0xec/0x3c8
>>> [ 6.419610] chrdev_open+0xb0/0x198
>>> [ 6.421093] do_dentry_open+0x200/0x310
>>> [ 6.422714] vfs_open+0x54/0x84
>>> [ 6.424054] path_openat+0x2dc/0xf04
>>> [ 6.425569] do_filp_open+0x68/0xd8
>>> [ 6.427044] do_sys_open+0x16c/0x224
>>> [ 6.428563] SyS_openat+0x10/0x18
>>> [ 6.429972] el0_svc_naked+0x30/0x34
>>> [ 6.431494] handlers:
>>> [ 6.432479] [<000000000e9fb4bb>] serial8250_interrupt
>>> [ 6.434597] Disabling IRQ #41
>>>
>>> This patch changes the lr state condition in lr_signals_eoi_mi() from
>>> invalid(Inactive) to active and pending to avoid this.
>>>
>>> I am not sure about the original design of the condition of
>>> invalid(active). So, This RFC is sent out for comments.
>>>
>>> Cc: Joey Zheng <yu.zheng@hxt-semitech.com>
>>> Signed-off-by: Shunyong Yang <shunyong.yang@hxt-semitech.com>
>>> ---
>>> virt/kvm/arm/vgic/vgic-v2.c | 4 ++--
>>> virt/kvm/arm/vgic/vgic-v3.c | 4 ++--
>>> 2 files changed, 4 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/virt/kvm/arm/vgic/vgic-v2.c b/virt/kvm/arm/vgic/vgic-v2.c
>>> index e9d840a75e7b..740ee9a5f551 100644
>>> --- a/virt/kvm/arm/vgic/vgic-v2.c
>>> +++ b/virt/kvm/arm/vgic/vgic-v2.c
>>> @@ -46,8 +46,8 @@ void vgic_v2_set_underflow(struct kvm_vcpu *vcpu)
>>>
>>> static bool lr_signals_eoi_mi(u32 lr_val)
>>> {
>>> - return !(lr_val & GICH_LR_STATE) && (lr_val & GICH_LR_EOI) &&
>>> - !(lr_val & GICH_LR_HW);
>>> + return !((lr_val & GICH_LR_STATE) ^ GICH_LR_STATE) &&
>>
>> That feels very wrong. You're now signalling the resampling in both
>> invalid and pending+active, and the latter state doesn't mean you've
>> EOIed anything. You're now over-signalling, and signalling the
>> wrong event.
>>
>>> + (lr_val & GICH_LR_EOI) && !(lr_val & GICH_LR_HW);
>>> }
>>>
>>> /*
>>> diff --git a/virt/kvm/arm/vgic/vgic-v3.c b/virt/kvm/arm/vgic/vgic-v3.c
>>> index 6b329414e57a..43111bba7af9 100644
>>> --- a/virt/kvm/arm/vgic/vgic-v3.c
>>> +++ b/virt/kvm/arm/vgic/vgic-v3.c
>>> @@ -35,8 +35,8 @@ void vgic_v3_set_underflow(struct kvm_vcpu *vcpu)
>>>
>>> static bool lr_signals_eoi_mi(u64 lr_val)
>>> {
>>> - return !(lr_val & ICH_LR_STATE) && (lr_val & ICH_LR_EOI) &&
>>> - !(lr_val & ICH_LR_HW);
>>> + return !((lr_val & ICH_LR_STATE) ^ ICH_LR_STATE) &&
>>> + (lr_val & ICH_LR_EOI) && !(lr_val & ICH_LR_HW);
>>> }
>>>
>>> void vgic_v3_fold_lr_state(struct kvm_vcpu *vcpu)
>>>
>>
>> Assuming I understand the issue correctly, I cannot really see how
>> to solve this without reintroducing EISR, which sucks majorly.
>>
>> I'll try to cook something shortly and we can all have a good
>> fight about how crap this is.
>
> Here's what I came up with. I don't really like it, but that's
> the least invasive this I could come up with. Please let me
> know if that helps with your test case. Note that I have only
> boot-tested this on a sample of 1 machine, so I don't expect this
> to be perfect.
>
> Also, any guideline on how to reproduce this would be much appreciated.
> I never used this mdev/mtty thing, so please bear with me.
>
> Thanks,
>
> M.
>
> From 66a7c4cfc1029b0169dd771e196e2876ba3f17b1 Mon Sep 17 00:00:00 2001
> From: Marc Zyngier <marc.zyngier@arm.com>
> Date: Thu, 8 Mar 2018 11:14:06 +0000
> Subject: [PATCH] KVM: arm/arm64: Do not rely on LR state to guess EOI MI
> status
>
> We so far rely on the LR state to decide whether the guest has
> EOI'd a level interrupt or not. While this looks like a good
> idea on the surface, it leads to a couple of annoying corner
> cases:
>
> Example 1: (P = Pending, A = Active, MI = Maintenance Interrupt)
> P -> guest IAR -> A -> exit/entry -> P+A -> guest EOI -> P -> MI
>
> The state is now pending, we've really EOI'd the interrupt, and
> yet lr_signals_eoi_mi() returns false, since the state is not 0.
> The result is that we won't signal anything on the corresponding
> irqfd, which people complain about. Meh.
>
> Example 2:
> P+A -> guest EOI -> P -> delayed MI -> guest IAR -> A -> MI fires
In that case aren't we acking the same IRQ occurence twice?
>
> Same issue: state isn't 0, and nothing happens.
>
> The core of the problem is that we can't decide on whether an
> interrupt has been EOId by just looking at the LR if we ever
> want to support the P+A state, as things do change behind our back.
>
> An alternative to dropping P+A is to bring back our friend EISR,
> which indicates which LRs have generated a MI. Instead of dragging
> the state around like we used to do, use it to clear the EOI bit
> from the in-memory copy, and use that as a predicate to find out
> if it fired or not.
>
> Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
> ---
> virt/kvm/arm/hyp/vgic-v2-sr.c | 8 ++++++++
> virt/kvm/arm/hyp/vgic-v3-sr.c | 6 ++++++
> virt/kvm/arm/vgic/vgic-v2.c | 3 +--
> virt/kvm/arm/vgic/vgic-v3.c | 3 +--
> 4 files changed, 16 insertions(+), 4 deletions(-)
>
> diff --git a/virt/kvm/arm/hyp/vgic-v2-sr.c b/virt/kvm/arm/hyp/vgic-v2-sr.c
> index 4fe6e797e8b3..475cb2d7fd33 100644
> --- a/virt/kvm/arm/hyp/vgic-v2-sr.c
> +++ b/virt/kvm/arm/hyp/vgic-v2-sr.c
> @@ -43,6 +43,11 @@ static void __hyp_text save_lrs(struct kvm_vcpu *vcpu, void __iomem *base)
> struct vgic_v2_cpu_if *cpu_if = &vcpu->arch.vgic_cpu.vgic_v2;
> int i;
> u64 used_lrs = vcpu->arch.vgic_cpu.used_lrs;
> + u64 eisr;
> +
> + eisr = readl_relaxed(base + GICH_EISR0);
> + if (unlikely(used_lrs > 32))
> + eisr |= (u64)readl_relaxed(base + GICH_EISR1) << 32;
>
> for (i = 0; i < used_lrs; i++) {
> if (cpu_if->vgic_elrsr & (1UL << i))
> @@ -50,6 +55,9 @@ static void __hyp_text save_lrs(struct kvm_vcpu *vcpu, void __iomem *base)
> else
> cpu_if->vgic_lr[i] = readl_relaxed(base + GICH_LR0 + (i * 4));
>
> + if ((cpu_if->vgic_lr[i] & GICH_LR_EOI) && !(eisr & (1UL << i)))
> + cpu_if->vgic_lr[i] &= ~GICH_LR_EOI;
> +
> writel_relaxed(0, base + GICH_LR0 + (i * 4));
> }
> }
> diff --git a/virt/kvm/arm/hyp/vgic-v3-sr.c b/virt/kvm/arm/hyp/vgic-v3-sr.c
> index b89ce5432214..2ce63d6740b0 100644
> --- a/virt/kvm/arm/hyp/vgic-v3-sr.c
> +++ b/virt/kvm/arm/hyp/vgic-v3-sr.c
> @@ -223,8 +223,10 @@ void __hyp_text __vgic_v3_save_state(struct kvm_vcpu *vcpu)
> if (used_lrs) {
> int i;
> u32 nr_pre_bits;
> + u32 eisr;
>
> cpu_if->vgic_elrsr = read_gicreg(ICH_ELSR_EL2);
> + eisr = read_gicreg(ICH_EISR_EL2);
>
> write_gicreg(0, ICH_HCR_EL2);
> val = read_gicreg(ICH_VTR_EL2);
> @@ -236,6 +238,10 @@ void __hyp_text __vgic_v3_save_state(struct kvm_vcpu *vcpu)
> else
> cpu_if->vgic_lr[i] = __gic_v3_get_lr(i);
>
> + if ((cpu_if->vgic_lr[i] & ICH_LR_EOI) &&
> + !(eisr & (1 << i)))
> + cpu_if->vgic_lr[i] &= ~ICH_LR_EOI;
> +
> __gic_v3_set_lr(0, i);
> }
>
> diff --git a/virt/kvm/arm/vgic/vgic-v2.c b/virt/kvm/arm/vgic/vgic-v2.c
> index e9d840a75e7b..0be616e4ee29 100644
> --- a/virt/kvm/arm/vgic/vgic-v2.c
> +++ b/virt/kvm/arm/vgic/vgic-v2.c
> @@ -46,8 +46,7 @@ void vgic_v2_set_underflow(struct kvm_vcpu *vcpu)
>
> static bool lr_signals_eoi_mi(u32 lr_val)
> {
> - return !(lr_val & GICH_LR_STATE) && (lr_val & GICH_LR_EOI) &&
> - !(lr_val & GICH_LR_HW);
> + return (lr_val & GICH_LR_EOI) && !(lr_val & GICH_LR_HW);
do we still need to test lr_val & GICH_LR_HW? Aren't LR_EOI and LR_HW
antagonist (maybe it is a reminder of architected timer stuff?)
> }
>
> /*
> diff --git a/virt/kvm/arm/vgic/vgic-v3.c b/virt/kvm/arm/vgic/vgic-v3.c
> index 6b329414e57a..c68352b8ed28 100644
> --- a/virt/kvm/arm/vgic/vgic-v3.c
> +++ b/virt/kvm/arm/vgic/vgic-v3.c
> @@ -35,8 +35,7 @@ void vgic_v3_set_underflow(struct kvm_vcpu *vcpu)
>
> static bool lr_signals_eoi_mi(u64 lr_val)
> {
> - return !(lr_val & ICH_LR_STATE) && (lr_val & ICH_LR_EOI) &&
> - !(lr_val & ICH_LR_HW);
> + return (lr_val & ICH_LR_EOI) && !(lr_val & ICH_LR_HW);
> }
>
> void vgic_v3_fold_lr_state(struct kvm_vcpu *vcpu)
>

Otherwise Looks good to me

Thanks

Eric

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