On 2022-04-29 12:54, Joao Martins wrote:
On 4/29/22 12:11, Robin Murphy wrote:
On 2022-04-28 22:09, Joao Martins wrote:
From: Kunkun Jiang <jiangkunkun@xxxxxxxxxx>
This detects BBML feature and if SMMU supports it, transfer BBMLx
quirk to io-pgtable.
BBML1 requires still marking PTE nT prior to performing a
translation table update, while BBML2 requires neither break-before-make
nor PTE nT bit being set. For dirty tracking it needs to clear
the dirty bit so checking BBML2 tells us the prerequisite. See SMMUv3.2
manual, section "3.21.1.3 When SMMU_IDR3.BBML == 2 (Level 2)" and
"3.21.1.2 When SMMU_IDR3.BBML == 1 (Level 1)"
You can drop this, and the dependencies on BBML elsewhere, until you get
round to the future large-page-splitting work, since that's the only
thing this represents. Not much point having the feature flags without
an actual implementation, or any users.
OK.
My thinking was that the BBML2 meant *also* that we don't need that break-before-make
thingie upon switching translation table entries. It seems that from what you
say, BBML2 then just refers to this but only on the context of switching between
hugepages/normal pages (?), not in general on all bits of the PTE (which we woud .. upon
switching from writeable-dirty to writeable-clean with DBM-set).
Yes, BBML is purely about swapping between a block (hugepage) entry and
a table representing the exact equivalent mapping.
A break-before-make procedure isn't required when just changing
permissions, and AFAICS it doesn't apply to changing the DBM bit either,
but as mentioned I think we could probably just not do that anyway.
Robin.
