Re: [PATCH v2 2/8] iommu/arm-smmu-v3: Use S2FWB when available

On Tue, Aug 27, 2024 at 12:48:13PM -0700, Nicolin Chen wrote:

Hi Jason,

On Tue, Aug 27, 2024 at 12:51:32PM -0300, Jason Gunthorpe wrote:

quoted

diff --git a/drivers/iommu/io-pgtable-arm.c b/drivers/iommu/io-pgtable-arm.c
index f5d9fd1f45bf49..9b3658aae21005 100644
--- a/drivers/iommu/io-pgtable-arm.c
+++ b/drivers/iommu/io-pgtable-arm.c

@@ -106,6 +106,18 @@
 #define ARM_LPAE_PTE_HAP_FAULT		(((arm_lpae_iopte)0) << 6)
 #define ARM_LPAE_PTE_HAP_READ		(((arm_lpae_iopte)1) << 6)
 #define ARM_LPAE_PTE_HAP_WRITE		(((arm_lpae_iopte)2) << 6)
+/*
+ * For !FWB these code to:
+ *  1111 = Normal outer write back cachable / Inner Write Back Cachable
+ *         Permit S1 to override
+ *  0101 = Normal Non-cachable / Inner Non-cachable
+ *  0001 = Device / Device-nGnRE
+ * For S2FWB these code:
+ *  0110 Force Normal Write Back
+ *  0101 Normal* is forced Normal-NC, Device unchanged
+ *  0001 Force Device-nGnRE
+ */
+#define ARM_LPAE_PTE_MEMATTR_FWB_WB	(((arm_lpae_iopte)0x6) << 2)

The other part looks good. Yet, would you mind sharing the location
that defines this 0x6 explicitly?

I'm looking at an older one ARM DDI 0487F.c

D5.5.5 Stage 2 memory region type and Cacheability attributes when FEAT_S2FWB is implemented

The text talks about the bits in the PTE, not relative to the MEMATTR
field, so 6 << 2 encodes to:

 543210
 011000

Then see table D5-40 Effect of bit[4] == 1 on Cacheability and Memory Type)

 Bit[5] = 0 = is RES0.
 Bit[4] = 1 = determines the interpretation of bits [3:2].
 Bits[3:2] == 10 == Normal Write-Back 

Here Bit means 'bit of the PTE' because the MemAttr does not have 5
bits.

Where it has the followings in D8.6.6:
 "For stage 2 translations, if FEAT_MTE_PERM is not implemented, then
  FEAT_S2FWB has all of the following effects on the MemAttr[3:2] bits:
   - MemAttr[3] is RES0.
   - The value of MemAttr[2] determines the interpretation of the
     MemAttr[1:0] bits.

And here the text switches from talking about the PTE bits to the
Field bits. MemAttr[3] == PTE[5], and the above text matches D5.5

The use of numbering schemes relative to the start of the field and
also relative to the PTE is tricky.

Jason