Hi !
I'm struggling a bit to understand the alignment constraints on the physical address we put in TTBR1_EL1. The ARM ARM v8 doesn't give a precise link in TTBR1_EL1 description to where is alignment is defined. For this post, I'm using a 4k granule everywhere.
From what I understand, bits [x-1,0] must be all 0, for x defined like in table
Table D4-25 Translation table entry addresses when using the 4KB translation granule
For exemple, for a T1SZ of 30, to map 16 GiB of memory, x would be 37 - 30 = 7 (4 x 4k pages)
for a T1SZ of 32, to map 4 GiB of memory, x would be 37 - 32 = 5 (1 x 4k pages)
However in the same table, we see that "BaseAddr[PAMax-1: x]:IA[y:30]:0b000" which would suggest that any look starting at level One needs to be 4K page align.
Could someone clarify the situation, and tell me which constraints I have to fullfuly for the value T1SZ of 30 and 32 ?
Best,
V.
I think this is important (DDI 487B.a):
"D7.2.102 TTBR1_EL1, Translation Table Base Register 1 (EL1)...
A translation table must be aligned to the size of the table, except that when using a translation table base address larger than 48 bits the minimum alignment of a table containing fewer than eight entries is 64 bytes."
Thank you for this reference, I just updated my copy of DDI 487 to DDI 487B.b and see if things are clearer
I'm still quite confused: the TTBR1_EL1 documentation you quote seems to state that for a 4 GiB virtual address space (that is 4 L1 Tables), I should align to 4 * 4096 = 16k -> 14 bits, but the same paragraph, at the end, state that the computation of the 'x' value is described before, and according to my understatement should be 37 - 32 = 5...
Anyone can confirm which interpretation is the right one ?
Note that the example "Example D4-1 Effect of an IA width of 35 bits when using the 4KB granule size" seems to go my way: with 35 bits of IA, TxSZ is 64 - 35 = 29 so we are using the first level lookup. So x is 37 - 29 = 8, and the example states that TTBRx_EL must provide bits [47,8]
Nevermind, both are the same, I got confused by the "size of the table"... In the 4 G/ 16 G scenarios, my 1st table are resp 4 and 16 entries, so they are very small, thus the aligment to x =5/12