Re: [PATCH v3 04/32] irqchip/gic-v3: Use SGIs without active state if offered

[Zenghui Yu]

Hi Marc,

On 2019/12/24 19:10, Marc Zyngier wrote:
> If running under control of a hypervisor that implements GICv4.1
> SGIs, allow the hypervisor to use them at the expense of loosing
> the Active state (which we don't care about for SGIs).
>
> This is trivially done by checking for GICD_TYPER2.nASSGIcap, and
> setting GICD_CTLR.nASSGIreq when enabling Group-1 interrupts.
>
> Signed-off-by: Marc Zyngier <maz@xxxxxxxxxx>
> ---
>   drivers/irqchip/irq-gic-v3.c       | 10 ++++++++--
>   include/linux/irqchip/arm-gic-v3.h |  2 ++
>   2 files changed, 10 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/irqchip/irq-gic-v3.c b/drivers/irqchip/irq-gic-v3.c
> index 640d4db65b78..624f351c0362 100644
> --- a/drivers/irqchip/irq-gic-v3.c
> +++ b/drivers/irqchip/irq-gic-v3.c
> @@ -724,6 +724,7 @@ static void __init gic_dist_init(void)
>   	unsigned int i;
>   	u64 affinity;
>   	void __iomem *base = gic_data.dist_base;
> +	u32 val;
>   
>   	/* Disable the distributor */
>   	writel_relaxed(0, base + GICD_CTLR);
> @@ -756,9 +757,14 @@ static void __init gic_dist_init(void)
>   	/* Now do the common stuff, and wait for the distributor to drain */
>   	gic_dist_config(base, GIC_LINE_NR, gic_dist_wait_for_rwp);
>   
> +	val = GICD_CTLR_ARE_NS | GICD_CTLR_ENABLE_G1A | GICD_CTLR_ENABLE_G1;
> +	if (gic_data.rdists.gicd_typer2 & GICD_TYPER2_nASSGIcap) {
> +		pr_info("Enabling SGIs without active state\n");
> +		val |= GICD_CTLR_nASSGIreq;
> +	}
> +
>   	/* Enable distributor with ARE, Group1 */
> -	writel_relaxed(GICD_CTLR_ARE_NS | GICD_CTLR_ENABLE_G1A | GICD_CTLR_ENABLE_G1,
> -		       base + GICD_CTLR);
> +	writel_relaxed(val, base + GICD_CTLR);
>   
>   	/*
>   	 * Set all global interrupts to the boot CPU only. ARE must be
> diff --git a/include/linux/irqchip/arm-gic-v3.h b/include/linux/irqchip/arm-gic-v3.h
> index 9dfe64189d99..72b69f4e6c7b 100644
> --- a/include/linux/irqchip/arm-gic-v3.h
> +++ b/include/linux/irqchip/arm-gic-v3.h
> +#define GICD_CTLR_nASSGIreq		(1U << 8)

> +#define GICD_TYPER2_nASSGIcap		(1U << 8)

I thought these two bits are newly added in the specification, which is
not available yet... until I've reached patch 29 and 30.

So they are actually some "kvm-implemented" bits and can only be used by
the KVM guests. I have two questions now:

1) As per the latest GIC specification, these two bits are Reserved
   (RAZ/WI) from host's perspective, which is good for now. But will
   they be (unexpectedly) used one day by the future architecture?

2) Only Linux guest will check and make use of these bits now. What if
   some non-Linux guests wants to run with KVM and use the GICv4.1 based
   vSGIs?  Their developers might have no interests reading at the KVM
   code... so what about plumbing some descriptions about these bits
   into somewhere in the documentation (or code)?  Like below, mostly
   copied from your commit messages:

---8<---

// Roughly writing, for the ease of reviewing the later patches.

No-Active-State SGIs (?) Related Field Descriptions
	--From Guest's Perspective

With GICv4.1, KVM exposes two single bit (in GICD_TYPER2 and GICD_CTLR
respectively) for guests, which can be used to probe the GICv4.1 based
SGIs support on hypervisor and choose whether guests want the good old
SGIs with an active state, or the new, HW-based ones that do not have
one.

GICD_TYPER2.nASSGIcap, bit [8]

    Indicates whether guests are running under control of a hypervisor
    that implements GICv4.1 SGIs, allow the hypervisor to use them at
    the expense of loosing the Active state.

    0b0    GICv4.1 SGIs capability is not offered by hypervisor.

    0b1    GICv4.1 SGIs capability is offered by hypervisor.

    This field is RO.


GICD_CTLR.nASSGIreq, bit [8]

    Indicates whether guests wants to use HW-based SGIs without Active
    state if the GICv4.1 SGI capability is offered by hypervisor.
    Hypervisor will then try to satisfy the request and switch between
    HW/SW delivered SGIs.

    0b0    If read, indicated that guest is using the old SW-emulated
           SGIs.
           If write, indicates that guest requests to use the old
           SW-emulated SGIs.

    0b1    If read, indicates that guest is using the new HW-based SGIs.
           If write, indicates that guest requests to use the new
           HW-based SGIs. If GICD_TYPER2.nASSGIcap is 0, the write has
           no effect.

    Changing this bit is UNPREDICTABLE if the Distributor is enabled.
    KVM may just treat this bit as RO when Distributor stays enabled.


See gic_dist_init() in drivers/irqchip/irq-gic-v3.c for an example that
how Linux guest (since 5.6?) makes use of these bits and benefits from
the GICv4.1 based vSGIs.

These two bits are Reserved (RAZ/WI) from host's perspective, which is
good for now.

---8<---


Thanks,

Zenghui