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12.5. Translation table configuration

In addition to storing individual translations within the TLB, you can configure the MMU to store translation tables in cacheable memory. This usually provides much faster access to tables than always reading from external memory. TCR_EL1 has additional fields that control this.

The additional fields specify the cacheability and shareability of translation tables for TTBR0 and TTBR1. The relevant fields are called SH0/1 Shareability, IRGN0/1 Inner Cacheable, and ORGN0/1 Outer Cacheable. Table 12.2 shows the permitted settings for cacheability.

Table 12.2. Cacheability settings
IRGN/ORGN bits for TTBR0/TTBR1Cacheable Property
00 Normal memory, Inner Non-cacheable
01 Normal memory, Inner Write-Back Write-Allocate Cacheable
10 Normal memory, Inner Write-Through Cacheable
11 Normal memory, Inner Write-Back no Write-Allocate Cacheable

The corresponding table for shareability of memory is associated with translation table walks. For a device or strongly-ordered memory region, the value is ignored.

Table 12.3. Memory shareability
SH0 bits[13:12] Shareability
01 Unpredictable
10 Outer shareable
11 Inner shareable

The attributes specified in the TCR_EL1 must be the same as those specified for the virtual memory region in which the translation tables are stored. Caching the translation tables is the normal default behavior.

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