linux-kernel - Re: [PATCH] drivers/io-pgtable-arm: Fix stage concatenation with 16K

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Message-ID: <888e6542-2aa4-43f1-b31e-79432c1ad199@arm.com>
Date: Fri, 15 Nov 2024 18:29:20 +0000
From: Robin Murphy <robin.murphy@....com>
To: Mostafa Saleh <smostafa@...gle.com>, linux-kernel@...r.kernel.org,
 iommu@...ts.linux.dev, linux-arm-kernel@...ts.infradead.org
Cc: will@...nel.org, joro@...tes.org, Daniel Mentz <danielmentz@...gle.com>
Subject: Re: [PATCH] drivers/io-pgtable-arm: Fix stage concatenation with 16K

On 2024-11-15 5:22 pm, Mostafa Saleh wrote:
> According to the Arm spec DDI0487 K.a, in:
> "Table D8-9 Implications of the effective minimum T0SZ value on the
> initial stage 2 lookup level"
> 
> Some combinations of granule and input size with stage-2 would
> require to use initial lookup levels that can only be achieved
> with concatenated PGDs.
> 
> There was one missing case in the current implementation for 16K,
> which is 40-bits.
> 
> Cc: Daniel Mentz <danielmentz@...gle.com>
> 
> Signed-off-by: Mostafa Saleh <smostafa@...gle.com>
> ---
>   drivers/iommu/io-pgtable-arm.c | 14 +++++++++++---
>   1 file changed, 11 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/iommu/io-pgtable-arm.c b/drivers/iommu/io-pgtable-arm.c
> index 0e67f1721a3d..9a57874a5cb8 100644
> --- a/drivers/iommu/io-pgtable-arm.c
> +++ b/drivers/iommu/io-pgtable-arm.c
> @@ -1044,10 +1044,18 @@ arm_64_lpae_alloc_pgtable_s2(struct io_pgtable_cfg *cfg, void *cookie)
>   		return NULL;
>   
>   	/*
> -	 * Concatenate PGDs at level 1 if possible in order to reduce
> -	 * the depth of the stage-2 walk.
> +	 * Some cases where concatenation is mandatory after de-ciphering RSRKBC
> +	 * in the Arm DDI0487 (K.a):
> +	 * - 40 bits with 4K: use 2 table at level 1 instead of level 0
> +	 * - 40 bits with 16K: use 16 tables at level 2 instead of level 1
> +	 * - 42 bits with 4K: use 8 tabels at level 1 instead of level 0
> +	 * - 48 bits with 16K: use 2 tabels at level 1 instead of level 0

This confused me, since per R_DXBSH, that last one is the only one which 
is actually mandatory in general; the others may be valid per R_PZFHQ 
and R_FBHPY. The additional R_SRKBC constraints come from the PA size, 
not the choice of T0SZ (and thus ultimately start_level) itself, so 
although I guess this probably works out true in practice based on how 
the SMMU drivers happen to behave today, it's none too obvious why.

Thanks,
Robin.

> +	 * Looking at the possible valid input size, that concludes to always
> +	 * use level 1 with concatentation if possible or at level 2 only
> +	 * with 16K.
>   	 */
> -	if (data->start_level == 0) {
> +	if ((data->start_level == 0) ||
> +	    ((data->start_level == 1) && (ARM_LPAE_GRANULE(data) == SZ_16K))) {
>   		unsigned long pgd_pages;
>   
>   		pgd_pages = ARM_LPAE_PGD_SIZE(data) / sizeof(arm_lpae_iopte);