linux-kernel - Re: [PATCH] irqchip/gic-v4: fix occasional VLPI drop

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Date:   Thu, 24 Jan 2019 10:33:35 +0000
From:   Marc Zyngier <marc.zyngier@....com>
To:     Heyi Guo <guoheyi@...wei.com>
Cc:     <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jason Cooper <jason@...edaemon.net>,
        <wanghaibin.wang@...wei.com>, <yangyingliang@...wei.com>
Subject: Re: [PATCH] irqchip/gic-v4: fix occasional VLPI drop

On Wed, 23 Jan 2019 08:59:08 +0000,
Heyi Guo <guoheyi@...wei.com> wrote:
> 
> 1. In current implementation, every VLPI will temporarily be mapped to
> the first CPU in system (normally CPU0) and then moved to the real
> scheduled CPU later.
> 
> 2. So there is a time window and a VLPI may be sent to CPU0 instead of
> the real scheduled vCPU, in a multi-CPU virtual machine.
> 
> 3. However, CPU0 may have not been scheduled as a virtual CPU after
> system boots up, so the value of its GICR_VPROPBASER is unknown at
> that moment.
> 
> 4. If the INTID of VLPI is larger than 2^(GICR_VPROPBASER.IDbits+1),
> while IDbits is also in unknown state, GIC will behave as if the VLPI
> is out of range and simply drop it, which results in interrupt missing
> in Guest.
> 
> As no code will clear GICR_VPROPBASER at runtime, we can safely
> initialize the IDbits field at boot time for each CPU to get rid of
> this issue.
> 
> We also clear Valid bit of GICR_VPENDBASER in case any ancient
> programming gets left in and causes memory corrupting.
> 
> Signed-off-by: Heyi Guo <guoheyi@...wei.com>
> Signed-off-by: Heyi Guo <heyi.guo@...aro.org>
> ---
>  drivers/irqchip/irq-gic-v3-its.c | 25 +++++++++++++++++++++++++
>  1 file changed, 25 insertions(+)
> 
> diff --git a/drivers/irqchip/irq-gic-v3-its.c b/drivers/irqchip/irq-gic-v3-its.c
> index 3365d44..634c8a7 100644
> --- a/drivers/irqchip/irq-gic-v3-its.c
> +++ b/drivers/irqchip/irq-gic-v3-its.c
> @@ -2145,6 +2145,31 @@ static void its_cpu_init_lpis(void)
>  	val |= GICR_CTLR_ENABLE_LPIS;
>  	writel_relaxed(val, rbase + GICR_CTLR);
>  
> +	if (gic_rdists->has_vlpis) {
> +		void __iomem *vlpi_base = gic_data_rdist_vlpi_base();
> +
> +		/*
> +		 * It's possible for CPU to receive VLPIs before it is
> +		 * sheduled as a vPE, especially for the first CPU, and the
> +		 * VLPI with INTID larger than 2^(IDbits+1) will be considered
> +		 * as out of range and dropped by GIC.
> +		 * So we initialize IDbits to known value to avoid VLPI drop.
> +		 */
> +		val = (LPI_NRBITS - 1) & GICR_VPROPBASER_IDBITS_MASK;
> +		pr_debug("GICv4: CPU%d: Init IDbits to 0x%llx for GICR_VPROPBASER\n",
> +			smp_processor_id(), val);
> +		gits_write_vpropbaser(val, vlpi_base + GICR_VPROPBASER);
> +
> +		/*
> +		 * Also clear Valid bit of GICR_VPENDBASER, in case some
> +		 * ancient programming gets left in and has possibility of
> +		 * corrupting memory.
> +		 */
> +		val = gits_read_vpendbaser(vlpi_base + GICR_VPENDBASER);
> +		val &= ~GICR_VPENDBASER_Valid;
> +		gits_write_vpendbaser(val, vlpi_base + GICR_VPENDBASER);

I'm afraid it is not enough to just write back with the valid bit
clear. You need to make sure Dirty has been cleared before continuing,
which is why I was asking you to reuse some bits of its_vpe_deschedule.

Something like this:

diff --git a/drivers/irqchip/irq-gic-v3-its.c b/drivers/irqchip/irq-gic-v3-its.c
index db20e992a40f..1808cb5f6a3e 100644
--- a/drivers/irqchip/irq-gic-v3-its.c
+++ b/drivers/irqchip/irq-gic-v3-its.c
@@ -2751,9 +2751,8 @@ static void its_vpe_schedule(struct its_vpe *vpe)
 	gits_write_vpendbaser(val, vlpi_base + GICR_VPENDBASER);
 }
 
-static void its_vpe_deschedule(struct its_vpe *vpe)
+static u64 its_clear_vpend_valid(void __iomem *vlpi_base)
 {
-	void __iomem *vlpi_base = gic_data_rdist_vlpi_base();
 	u32 count = 1000000;	/* 1s! */
 	bool clean;
 	u64 val;
@@ -2773,7 +2772,17 @@ static void its_vpe_deschedule(struct its_vpe *vpe)
 		}
 	} while (!clean && count);
 
-	if (unlikely(!clean && !count)) {
+	return val;
+}
+
+static void its_vpe_deschedule(struct its_vpe *vpe)
+{
+	void __iomem *vlpi_base = gic_data_rdist_vlpi_base();
+	u64 val;
+
+	val = its_clear_vpend_valid(vlpi_base);
+
+	if (unlikely(val & GICR_VPENDBASER_Dirty)) {
 		pr_err_ratelimited("ITS virtual pending table not cleaning\n");
 		vpe->idai = false;
 		vpe->pending_last = true;

And you can the rewrite the above GICR_VPENDBASER_Valid clearing as:

	val = its_clear_vpend_valid(vlpi_base);
	WARN_ON(val & GICR_VPENDBASER_Dirty);

We'll at least get a warning if something goes wrong.

Thanks,

	M.

-- 
Jazz is not dead, it just smell funny.