linux-kernel - Re: [PATCH 04/35] irqchip/gic-v3: Detect GICv4.1 supporting RVPEID

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Date:   Tue, 24 Sep 2019 11:24:14 +0100
From:   Andrew Murray <andrew.murray@....com>
To:     Marc Zyngier <maz@...nel.org>
Cc:     kvmarm@...ts.cs.columbia.edu, linux-kernel@...r.kernel.org,
        Eric Auger <eric.auger@...hat.com>,
        James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jason Cooper <jason@...edaemon.net>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>
Subject: Re: [PATCH 04/35] irqchip/gic-v3: Detect GICv4.1 supporting RVPEID

On Mon, Sep 23, 2019 at 07:25:35PM +0100, Marc Zyngier wrote:
> GICv4.1 supports the RVPEID ("Residency per vPE ID"), which allows for
> a much efficient way of making virtual CPUs resident (to allow direct
> injection of interrupts).
> 
> The functionnality needs to be discovered on each and every redistributor
> in the system, and disabled if the settings are inconsistent.
> 
> Signed-off-by: Marc Zyngier <maz@...nel.org>
> ---
>  drivers/irqchip/irq-gic-v3.c       | 21 ++++++++++++++++++---
>  include/linux/irqchip/arm-gic-v3.h |  2 ++
>  2 files changed, 20 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/irqchip/irq-gic-v3.c b/drivers/irqchip/irq-gic-v3.c
> index 422664ac5f53..0b545e2c3498 100644
> --- a/drivers/irqchip/irq-gic-v3.c
> +++ b/drivers/irqchip/irq-gic-v3.c
> @@ -849,8 +849,21 @@ static int __gic_update_rdist_properties(struct redist_region *region,
>  					 void __iomem *ptr)
>  {
>  	u64 typer = gic_read_typer(ptr + GICR_TYPER);
> +
>  	gic_data.rdists.has_vlpis &= !!(typer & GICR_TYPER_VLPIS);
> -	gic_data.rdists.has_direct_lpi &= !!(typer & GICR_TYPER_DirectLPIS);
> +
> +	/* RVPEID implies some form of DirectLPI, no matter what the doc says... :-/ */

I think the doc says, RVPEID is *always* 1 for GICv4.1 (and presumably beyond)
and when RVPEID==1 then DirectLPI is *always* 0 - but that's OK because for
GICv4.1 support for direct LPIs is mandatory.

> +	gic_data.rdists.has_rvpeid &= !!(typer & GICR_TYPER_RVPEID);
> +	gic_data.rdists.has_direct_lpi &= (!!(typer & GICR_TYPER_DirectLPIS) |
> +					   gic_data.rdists.has_rvpeid);
> +
> +	/* Detect non-sensical configurations */
> +	if (WARN_ON_ONCE(gic_data.rdists.has_rvpeid && !gic_data.rdists.has_vlpis)) {

How feasible is the following suitation? All the redistributors in the system has
vlpis=0, and only the first redistributor has rvpeid=1 (with the remaining ones
rvpeid=0). If we evaluate this WARN_ON_ONCE on each call to
__gic_update_rdist_properties we end up without direct LPI support, however if we
evaluated this after iterating through all the redistributors then we'd end up
with direct LPI support and a non-essential WARN.

Should we do the WARN after iterating through all the redistributors once we
know what the final values of these flags will be, perhaps in
gic_update_rdist_properties?

> +		gic_data.rdists.has_direct_lpi = false;
> +		gic_data.rdists.has_vlpis = false;
> +		gic_data.rdists.has_rvpeid = false;
> +	}
> +
>  	gic_data.ppi_nr = min(GICR_TYPER_NR_PPIS(typer), gic_data.ppi_nr);
>  
>  	return 1;
> @@ -863,9 +876,10 @@ static void gic_update_rdist_properties(void)
>  	if (WARN_ON(gic_data.ppi_nr == UINT_MAX))
>  		gic_data.ppi_nr = 0;
>  	pr_info("%d PPIs implemented\n", gic_data.ppi_nr);
> -	pr_info("%sVLPI support, %sdirect LPI support\n",
> +	pr_info("%sVLPI support, %sdirect LPI support, %sRVPEID support\n",
>  		!gic_data.rdists.has_vlpis ? "no " : "",
> -		!gic_data.rdists.has_direct_lpi ? "no " : "");
> +		!gic_data.rdists.has_direct_lpi ? "no " : "",
> +		!gic_data.rdists.has_rvpeid ? "no " : "");
>  }
>  
>  /* Check whether it's single security state view */
> @@ -1546,6 +1560,7 @@ static int __init gic_init_bases(void __iomem *dist_base,
>  						 &gic_data);
>  	irq_domain_update_bus_token(gic_data.domain, DOMAIN_BUS_WIRED);
>  	gic_data.rdists.rdist = alloc_percpu(typeof(*gic_data.rdists.rdist));
> +	gic_data.rdists.has_rvpeid = true;
>  	gic_data.rdists.has_vlpis = true;
>  	gic_data.rdists.has_direct_lpi = true;
>  
> diff --git a/include/linux/irqchip/arm-gic-v3.h b/include/linux/irqchip/arm-gic-v3.h
> index 5cc10cf7cb3e..b34e0c113697 100644
> --- a/include/linux/irqchip/arm-gic-v3.h
> +++ b/include/linux/irqchip/arm-gic-v3.h
> @@ -234,6 +234,7 @@
>  #define GICR_TYPER_VLPIS		(1U << 1)
>  #define GICR_TYPER_DirectLPIS		(1U << 3)
>  #define GICR_TYPER_LAST			(1U << 4)
> +#define GICR_TYPER_RVPEID		(1U << 7)
>  
>  #define GIC_V3_REDIST_SIZE		0x20000
>  
> @@ -613,6 +614,7 @@ struct rdists {
>  	u64			flags;
>  	u32			gicd_typer;
>  	bool			has_vlpis;
> +	bool			has_rvpeid;
>  	bool			has_direct_lpi;
>  };
>  
> -- 
> 2.20.1
>