We have a fair amount of code in QEMU which is helper functions written in C which make guest memory accesses -- the most common case being page table walk code.
Generally in CPU architectures the accesses by things like the page table walker have some atomicity guarantees. For instance Arm requires that reads of translation tables are single-copy-atomic (v8A Arm ARM rev B.a section B2.2.1). If we supported the hardware management of the Access flag (we don't currently) that wolud have to be an atomic read-modify-write of the descriptor memory (section D4.4.9). What's the right API in QEMU to do these guest memory accesses with the required atomicity guarantees? Currently target/arm uses address_space_ldq_le(), which turns into a ldq_he_p(), which is a memcpy(), which I don't think is guaranteed to be a 64-bit load. And I don't think we have an accessor function for doing an atomic read-modify-write, do we? [I think one of the PPC maintainers first brought this issue up on IRC some while back, but I'm afraid I can't remember who that was.] thanks -- PMM
