On 3/18/25 17:02, Pierrick Bouvier wrote:
On 3/18/25 14:31, Richard Henderson wrote:
The implementation of cpu_mmu_index was split between cpu-common.h
and cpu-all.h, depending on CONFIG_USER_ONLY. We already have the
plumbing common to user and system mode. Using MMU_USER_IDX
requires the cpu.h for a specific target, and so is restricted to
when we're compiling per-target.
A side question: Why is MMU_USER_IDX different depending on architecture?
I'm trying to understand why (and by what) previous indexes are reserved when MMU_USER_IDX
is not zero.
Depends on the translator, but often: cpu_mmu_index may be encoded into tb_flags, and the
translator *also* uses this encoding to determine the priv state. So if, in user-only
mode, we fail to encode MMU_USER_IDX into tb_flags, we'll get incorrect priv checks in the
translator and fail to raise SIGILL for privledged operations.
Depending on the target, the mmu_index space may be quite complicated, with various
meanings assigned to various bits. Thus "0" may not be reasonable for MMU_USER_IDX.
See, for instance, enum ARMMMUIdx or ppc hreg_compute_hflags_value().
(Both of which, amusingly, use MMU_USER_IDX 0; neither here nor there.)
r~
