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

From: Marc Zyngier <[email-masked]>
Date: Mon, 23 Nov 2020 09:01:04 +0000

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.

> +	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.

> +		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.

> +
> +	*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).

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.

>  	.irq_set_irqchip_state	= its_irq_set_irqchip_state,
>  	.irq_retrigger		= its_irq_retrigger,
>  	.irq_set_vcpu_affinity	= its_irq_set_vcpu_affinity,

Thanks,

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

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