linux-kernel - Re: [PATCH v4 15/24] iommu/io-pgtable: Remove non-strict quirk

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Date:   Tue, 24 Aug 2021 15:25:38 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
        Linux IOMMU <iommu@...ts.linux-foundation.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        suravee.suthikulpanit@....com, baolu.lu@...ux.intel.com,
        John Garry <john.garry@...wei.com>,
        Doug Anderson <dianders@...omium.org>, rajatja@...gle.com,
        chenxiang <chenxiang66@...ilicon.com>,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>
Subject: Re: [PATCH v4 15/24] iommu/io-pgtable: Remove non-strict quirk

Hi Robin,

On Wed, Aug 11, 2021 at 2:24 PM Robin Murphy <robin.murphy@....com> wrote:
> IO_PGTABLE_QUIRK_NON_STRICT was never a very comfortable fit, since it's
> not a quirk of the pagetable format itself. Now that we have a more
> appropriate way to convey non-strict unmaps, though, this last of the
> non-quirk quirks can also go, and with the flush queue code also now
> enforcing its own ordering we can have a lovely cleanup all round.
>
> Signed-off-by: Robin Murphy <robin.murphy@....com>

Thanks for your patch, which is now commit a8e5f04458c4e496
("iommu/io-pgtable: Remove non-strict quirk") in iommu/next.

> --- a/drivers/iommu/io-pgtable-arm-v7s.c
> +++ b/drivers/iommu/io-pgtable-arm-v7s.c
> @@ -700,14 +700,7 @@ static size_t __arm_v7s_unmap(struct arm_v7s_io_pgtable *data,
>                                                 ARM_V7S_BLOCK_SIZE(lvl + 1));
>                                 ptep = iopte_deref(pte[i], lvl, data);
>                                 __arm_v7s_free_table(ptep, lvl + 1, data);
> -                       } else if (iop->cfg.quirks & IO_PGTABLE_QUIRK_NON_STRICT) {
> -                               /*
> -                                * Order the PTE update against queueing the IOVA, to
> -                                * guarantee that a flush callback from a different CPU
> -                                * has observed it before the TLBIALL can be issued.
> -                                */
> -                               smp_wmb();
> -                       } else {
> +                       } else if (!gather->queued) {

If CONFIG_IOMMU_API=n:

error: ‘struct iommu_iotlb_gather’ has no member named ‘queued’

This can be reproduced using e.g. shmobile_defconfig with
    CONFIG_IOMMU_SUPPORT=y
    CONFIG_IOMMU_IO_PGTABLE_ARMV7S=y


>                                 io_pgtable_tlb_add_page(iop, gather, iova, blk_size);
>                         }
>                         iova += blk_size;

> --- a/drivers/iommu/io-pgtable-arm.c
> +++ b/drivers/iommu/io-pgtable-arm.c
> @@ -638,14 +638,7 @@ static size_t __arm_lpae_unmap(struct arm_lpae_io_pgtable *data,
>                                 io_pgtable_tlb_flush_walk(iop, iova + i * size, size,
>                                                           ARM_LPAE_GRANULE(data));
>                                 __arm_lpae_free_pgtable(data, lvl + 1, iopte_deref(pte, data));
> -                       } else if (iop->cfg.quirks & IO_PGTABLE_QUIRK_NON_STRICT) {
> -                               /*
> -                                * Order the PTE update against queueing the IOVA, to
> -                                * guarantee that a flush callback from a different CPU
> -                                * has observed it before the TLBIALL can be issued.
> -                                */
> -                               smp_wmb();
> -                       } else {
> +                       } else if (!gather->queued) {

If CONFIG_IOMMU_API=n:

error: ‘struct iommu_iotlb_gather’ has no member named ‘queued’

This can be reproduced using e.g. shmobile_defconfig with
    CONFIG_IOMMU_SUPPORT=y
    CONFIG_IOMMU_IO_PGTABLE_LPAE=y

>                                 io_pgtable_tlb_add_page(iop, gather, iova + i * size, size);
>                         }
>

Perhaps "select IOMMU_API" should be added (moved from individual
drivers) to both IOMMU_IO_PGTABLE_ARMV7S and IOMMU_IO_PGTABLE_LPAE?
Or iommu_iotlb_gather.queued should not be accessed here, or the
access wrapped into a static inline helper function with a dummy for
the CONFIG_IOMMU_API=n case?

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds