linux-kernel - Re: [RESPIN PATCH] irqchip/gic-v3-its:Fix GICv4.1 needless VSYNC after unmap VPE

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Message-ID: <86cyr6u58r.wl-maz@kernel.org>
Date: Wed, 03 Apr 2024 11:09:56 +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: [RESPIN PATCH] irqchip/gic-v3-its:Fix GICv4.1 needless VSYNC after unmap VPE

Thanks for respinning this.

A few remarks:

The subject line could be improved. Something like:

"irqchip/gic-v4: Don't issue a VSYNC after VMAPP with V=0"

On Wed, 03 Apr 2024 09:35:56 +0100,
t00849498 <tangnianyao@...wei.com> wrote:
> 
> From: Nianyao Tang <tangnianyao@...wei.com>
> 
> Quote from GIC spec 5.3.19, a VMAPP with {V, Alloc}=={0, x}
> is self-synchronizing, This means the ITS command queue does not
> show the command as consumed until all of its effects are completed.

Since this is a direct quote, make it clear that it is so.

>
> We don't need VSYNC to guarantee unmap finish. And VSYNC after unmap VPE
> will reach an invalid vpe table entry, which may trigger exception
> like SError or RAS. Let's fix it.

This should be much stronger. It's not that we don't need VSYNC. It is
that VSYNC is actively wrong. I suggest that you rewrite the commit
message along these lines:

<msg>
As per the GICv4.1 spec (Arm IHI 0069H, 5.3.19):

"A VMAPP with {V, Alloc}=={0, x} is self-synchronizing. This means the
 ITS command queue does not show the command as consumed until all of
 its effects are completed."

Furthermore, VSYNC is allowed to deliver an SError when referencing a
non existent VPE.

By these definitions, a VMAPP followed by a VSYNC is a bug, as the
later references a VPE that has been unmapped by the former.

Fix it by eliding the VSYNC in this scenario.
</msg>

> 
> Signed-off-by: Nianyao Tang <tangnianyao@...wei.com>

Please also add:

Fixes: 64edfaa9a234 ("irqchip/gic-v4.1: Implement the v4.1 flavour of VMAPP")

With the above fixed:

Reviewed-by: Marc Zyngier <maz@...nel.org>

Thanks,

	M.

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