linux-kernel - Re: [PATCH] irqchip/gic-v4.1:Check whether indirect table is supported in allocate_vpe_l1

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Date: Wed, 15 May 2024 10:34:46 +0100
From: Marc Zyngier <maz@...nel.org>
To: Tangnianyao <tangnianyao@...wei.com>
Cc: <tglx@...utronix.de>,
	<linux-kernel@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>,
	<guoyang2@...wei.com>,
	<wangwudi@...ilicon.com>,
	jiangkunkun <jiangkunkun@...wei.com>
Subject: Re: [PATCH] irqchip/gic-v4.1:Check whether indirect table is supported in allocate_vpe_l1_table

On Wed, 15 May 2024 09:56:10 +0100,
Tangnianyao <tangnianyao@...wei.com> wrote:
> 
> 
> 
> On 1/22/2024 22:02, Marc Zyngier wrote:
> > On Mon, 22 Jan 2024 13:13:09 +0000,
> > Tangnianyao <tangnianyao@...wei.com> wrote:
> >> On 1/22/2024 17:00, Marc Zyngier wrote:
> >>> [Fixing the LKML address, which has bits of Stephan's address embedded
> >>> in it...]
> >>>
> >>> On Mon, 22 Jan 2024 16:06:07 +0000,
> >>> Nianyao Tang <tangnianyao@...wei.com> wrote:
> >>>> In allocate_vpe_l1_table, when we fail to inherit VPE table from other
> >>>> redistributors or ITSs, and we allocate a new vpe table for current common 
> >>>> affinity field without checking whether indirect table is supported.
> >>>> Let's fix it.
> >>> Is there an actual implementation that doesn't support the indirect
> >>> property for the VPE table? I know this is allowed for consistency
> >>> with the original revision of the architecture, but I never expected
> >>> an actual GICv4.1 implementation to be *that* bad.
> >>>
> >>> If that's the case, I'm a bit puzzled/worried.
> >> I met this problem in a developing implementation and find it's allowed by GIC spec.
> >> In such environment,  in a common affinity field with only redistributors and without
> >> any ITS in it, forcing its_vpe_id_alloc to allocate a large vpeid(like 65000), and there
> >> comes an error message "VPE IRQ allocation failure". It originally comes from
> >> allocate_vpe_l2_table, reading GICR_VPROPBASER with GICR_VPROPBASER_4_1_SIZE=1
> >> and GICR_VPROPBASER_4_1_INDIRECT=0.
> > Really, you should get your HW engineers to fix their GIC
> > implementation.  I'm OK with working around this issue for
> > completeness, but shipping such an implementation would be a mistake.
> >
> > [...]
> >
> >> I have another question here. The max number of pages  for GITS_BASER
> >> and GICR_VPROPBASER is different here, while GITS_BASER.Size is
> >> bit[7:0] with max 256, and GICR_4_1_VPROPBASER.Size is bit[6:0] with max 128.
> >> Kernel usually probe ITS basers first and then probe GICR_4_1_VPROPBASER in
> >> a common affinity group. Maybe we need to check this in "inherit_vpe_l1_table_from_its" ?
> > This is because GITS_BASER[] is generic (also works for devices and
> > collections), while GICR_VPROPBASER is tailored to the VPE table which
> > is usually smaller.
> >
> > I would expect that GICD_TYPER2.VID reports something that cannot
> > result in something going wrong (in this case, the L1 allocation
> > cannot be more than 128 pages).
> >
> > Overall, the kernel isn't a validation suite for the HW, and we expect
> > it to have some level of sanity. So if none of this is in shipping HW
> > but only in some model with crazy parameters, I don't think we should
> > go out of our way to support it.
> >
> > Thanks,
> >
> > 	M.
> >
> 
> Hi Marc,
> Friendly ping. Do we have plan to fix this problem on kernel, or any other plan ?

Hi Nianyao,

My earlier question still stand: is this something that affects a
shipping implementation? If not, then I don't think we should support
this upstream, as this doesn't seem like a realistic configuration.

If your employer has actually built this (which I still consider as a
mistake), then we can add the workaround I suggested.

Thanks,

	M.

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