Subject: Re: [RFC PATCH v1 1/4] irqchip/gic-v4.1: Plumb get_irqchip_state VLPI callback

From: Marc Zyngier <[email-masked]>
Date: Tue, 24 Nov 2020 08:08:10 +0000

On 2020-11-24 07:38, Shenming Lu wrote:
> On 2020/11/23 17:01, Marc Zyngier wrote:
>> On 2020-11-23 06:54, Shenming Lu wrote:
>>> From: Zenghui Yu <[email-masked]>
>>> 
>>> Up to now, the irq_get_irqchip_state() callback of its_irq_chip
>>> leaves unimplemented since there is no architectural way to get
>>> the VLPI's pending state before GICv4.1. Yeah, there has one in
>>> v4.1 for VLPIs.
>>> 
>>> With GICv4.1, after unmapping the vPE, which cleans and invalidates
>>> any caching of the VPT, we can get the VLPI's pending state by
>> 
>> This is a crucial note: without this unmapping and invalidation,
>> the pending bits are not generally accessible (they could be cached
>> in a GIC private structure, cache or otherwise).
>> 
>>> peeking at the VPT. So we implement the irq_get_irqchip_state()
>>> callback of its_irq_chip to do it.
>>> 
>>> Signed-off-by: Zenghui Yu <[email-masked]>
>>> Signed-off-by: Shenming Lu <[email-masked]>
>>> ---
>>>  drivers/irqchip/irq-gic-v3-its.c | 38 
>>> ++++++++++++++++++++++++++++++++
>>>  1 file changed, 38 insertions(+)
>>> 
>>> diff --git a/drivers/irqchip/irq-gic-v3-its.c 
>>> b/drivers/irqchip/irq-gic-v3-its.c
>>> index 0fec31931e11..287003cacac7 100644
>>> --- a/drivers/irqchip/irq-gic-v3-its.c
>>> +++ b/drivers/irqchip/irq-gic-v3-its.c
>>> @@ -1695,6 +1695,43 @@ static void its_irq_compose_msi_msg(struct
>>> irq_data *d, struct msi_msg *msg)
>>>      iommu_dma_compose_msi_msg(irq_data_get_msi_desc(d), msg);
>>>  }
>>> 
>>> +static bool its_peek_vpt(struct its_vpe *vpe, irq_hw_number_t hwirq)
>>> +{
>>> +    int mask = hwirq % BITS_PER_BYTE;
>> 
>> nit: this isn't a mask, but a shift instead. BIT(hwirq % BPB) would 
>> give
>> you a mask.
> 
> Ok, I will correct it.
> 
>> 
>>> +    void *va;
>>> +    u8 *pt;
>>> +
>>> +    va = page_address(vpe->vpt_page);
>>> +    pt = va + hwirq / BITS_PER_BYTE;
>>> +
>>> +    return !!(*pt & (1U << mask));
>>> +}
>>> +
>>> +static int its_irq_get_irqchip_state(struct irq_data *d,
>>> +                     enum irqchip_irq_state which, bool *val)
>>> +{
>>> +    struct its_device *its_dev = irq_data_get_irq_chip_data(d);
>>> +    struct its_vlpi_map *map = get_vlpi_map(d);
>>> +
>>> +    if (which != IRQCHIP_STATE_PENDING)
>>> +        return -EINVAL;
>>> +
>>> +    /* not intended for physical LPI's pending state */
>>> +    if (!map)
>>> +        return -EINVAL;
>>> +
>>> +    /*
>>> +     * In GICv4.1, a VMAPP with {V,Alloc}=={0,1} cleans and 
>>> invalidates
>>> +     * any caching of the VPT associated with the vPEID held in the 
>>> GIC.
>>> +     */
>>> +    if (!is_v4_1(its_dev->its) || 
>>> atomic_read(&map->vpe->vmapp_count))
>> 
>> It isn't clear to me what prevents this from racing against a mapping 
>> of
>> the VPE. Actually, since we only hold the LPI irqdesc lock, I'm pretty 
>> sure
>> nothing prevents it.
> 
> Yes, should have the vmovp_lock held?

That's not helping because of the VPE activation.

> And is it necessary to also hold this lock in
> its_vpe_irq_domain_activate/deactivate?

Well, you'd need that, but that's unnecessary complex AFAICT.

> 
>> 
>>> +        return -EACCES;
>> 
>> I can sort of buy EACCESS for a VPE that is currently mapped, but a 
>> non-4.1
>> ITS should definitely return EINVAL.
> 
> Alright, EINVAL looks better.
> 
>> 
>>> +
>>> +    *val = its_peek_vpt(map->vpe, map->vintid);
>>> +
>>> +    return 0;
>>> +}
>>> +
>>>  static int its_irq_set_irqchip_state(struct irq_data *d,
>>>                       enum irqchip_irq_state which,
>>>                       bool state)
>>> @@ -1975,6 +2012,7 @@ static struct irq_chip its_irq_chip = {
>>>      .irq_eoi        = irq_chip_eoi_parent,
>>>      .irq_set_affinity    = its_set_affinity,
>>>      .irq_compose_msi_msg    = its_irq_compose_msi_msg,
>>> +    .irq_get_irqchip_state    = its_irq_get_irqchip_state,
>> 
>> My biggest issue with this is that it isn't a reliable interface.
>> It happens to work in the context of KVM, because you make sure it
>> is called at the right time, but that doesn't make it safe in general
>> (anyone with the interrupt number is allowed to call this at any 
>> time).
> 
> We check the vmapp_count in it to ensure the unmapping of the vPE, and
> let the caller do the unmapping (they should know whether it is the 
> right
> time). If the unmapping is not done, just return a failure.

And without guaranteeing mutual exclusion against a concurrent VMAPP,
checking the vmapp_count means nothing (you need the lock described
above, and start sprinkling it around in other places as well).

>> 
>> Is there a problem with poking at the VPT page from the KVM side?
>> The code should be exactly the same (maybe simpler even), and at least
>> you'd be guaranteed to be in the correct context.
> 
> Yeah, that also seems a good choice.
> If you prefer it, we can try to realize it in v2.

I'd certainly prefer that. Let me know if you spot any implementation
issue with that.

Thanks,

         M.
-- 
Jazz is not dead. It just smells funny...

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