linux-kernel - Re: [PATCH] irqchip/gic-v3-its: Don't need VSYNC if VMAPP with {V, Alloc}=={0, x}

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Message-ID: <86jzlgt014.wl-maz@kernel.org>
Date: Tue, 02 Apr 2024 13:35:35 +0100
From: Marc Zyngier <maz@...nel.org>
To: t00849498 <tangnianyao@...wei.com>
Cc: <tglx@...utronix.de>,
	<linux-arm-kernel@...ts.infradead.org>,
	<linux-kernel@...r.kernel.org>,
	<guoyang2@...wei.com>,
	<wangwudi@...ilicon.com>
Subject: Re: [PATCH] irqchip/gic-v3-its: Don't need VSYNC if VMAPP with {V, Alloc}=={0, x}

On Tue, 02 Apr 2024 12:41:47 +0100,
t00849498 <tangnianyao@...wei.com> wrote:
> 
> From GIC spec, a VMAPP with {V, Alloc}=={0, x} is self-synchronizing,

It'd be nice to quote the part of the spec (5.3.19).

> This means the ITS command queue does not show the command as
> consumed until all of its effects are completed. A VSYNC with unmapped
> vpeid is not needed.
> 
> Signed-off-by: t00849498 <tangnianyao@...wei.com>

Previous contributions with the same email address had the name
"Nianyao Tang" associated with it. Was it wrong in the past? Or is the
above wrong?

> ---
>  drivers/irqchip/irq-gic-v3-its.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/drivers/irqchip/irq-gic-v3-its.c b/drivers/irqchip/irq-gic-v3-its.c
> index fca888b36680..a0ca5dcbb245 100644
> --- a/drivers/irqchip/irq-gic-v3-its.c
> +++ b/drivers/irqchip/irq-gic-v3-its.c
> @@ -789,6 +789,7 @@ static struct its_vpe *its_build_vmapp_cmd(struct its_node *its,
>  	unsigned long vpt_addr, vconf_addr;
>  	u64 target;
>  	bool alloc;
> +	bool unmap_v4_1 = !desc->its_vmapp_cmd.valid && is_v4_1(its);
>
>  	its_encode_cmd(cmd, GITS_CMD_VMAPP);
>  	its_encode_vpeid(cmd, desc->its_vmapp_cmd.vpe->vpe_id);
> @@ -832,6 +833,9 @@ static struct its_vpe *its_build_vmapp_cmd(struct its_node *its,
>  out:
>  	its_fixup_cmd(cmd);
>  
> +	if (unmap_v4_1)
> +		return NULL;
> +
>  	return valid_vpe(its, desc->its_vmapp_cmd.vpe);
>  }
>  

This is a bit ugly. We already have a whole block dedicated to
handling VMAPP with V=0 and GICv4.1, and it'd be more readable to keep
all that code together. Something like the untested patch below.

Thanks,

	M.

diff --git a/drivers/irqchip/irq-gic-v3-its.c b/drivers/irqchip/irq-gic-v3-its.c
index fca888b36680..2a537cbfcb07 100644
--- a/drivers/irqchip/irq-gic-v3-its.c
+++ b/drivers/irqchip/irq-gic-v3-its.c
@@ -786,6 +786,7 @@ static struct its_vpe *its_build_vmapp_cmd(struct its_node *its,
 					   struct its_cmd_block *cmd,
 					   struct its_cmd_desc *desc)
 {
+	struct its_vpe *vpe = valid_vpe(its, desc->its_vmapp_cmd.vpe);
 	unsigned long vpt_addr, vconf_addr;
 	u64 target;
 	bool alloc;
@@ -798,6 +799,11 @@ static struct its_vpe *its_build_vmapp_cmd(struct its_node *its,
 		if (is_v4_1(its)) {
 			alloc = !atomic_dec_return(&desc->its_vmapp_cmd.vpe->vmapp_count);
 			its_encode_alloc(cmd, alloc);
+			/*
+			 * Unmapping a VPE is self-synchronizing on GICv4.1,
+			 * no need to issue a VSYNC.
+			 */
+			vpe = NULL;
 		}
 
 		goto out;
@@ -832,7 +838,7 @@ static struct its_vpe *its_build_vmapp_cmd(struct its_node *its,
 out:
 	its_fixup_cmd(cmd);
 
-	return valid_vpe(its, desc->its_vmapp_cmd.vpe);
+	return vpe;
 }
 
 static struct its_vpe *its_build_vmapti_cmd(struct its_node *its,

-- 
Without deviation from the norm, progress is not possible.