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

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Message-ID: <20241115172235.1493328-1-smostafa@google.com>
Date: Fri, 15 Nov 2024 17:22:35 +0000
From: Mostafa Saleh <smostafa@...gle.com>
To: linux-kernel@...r.kernel.org, iommu@...ts.linux.dev, 
	linux-arm-kernel@...ts.infradead.org
Cc: will@...nel.org, robin.murphy@....com, joro@...tes.org, 
	Mostafa Saleh <smostafa@...gle.com>, Daniel Mentz <danielmentz@...gle.com>
Subject: [PATCH] drivers/io-pgtable-arm: Fix stage concatenation with 16K

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
+	 * 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);
-- 
2.47.0.338.g60cca15819-goog