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A question about the access flag fault

zhaoshijun

Hi, everyone.

 

The CPU will receive a access flag fault if we set the access flag = 0. My question is  if we always set the access flag to be zero in the fault handler, does the CPU get the value in the memory? For example, the instruction "ldr rd, address" loads the value in the address to rd, and we set the access flag of the PTE to be 0. In the software access fault handler, we still set the access flag to be 0.  My understanding is that after the fault handler, the PC register will point to the instruction, and the access flag fault will be triggered again, so the CPU won't load the memory in "address" to rd register.

However, I see a answer from the discussion in this community (https://community.arm.com/processors/f/discussions/5413/the-usage-of-access-flag-bit-in-translation-table-descriptor) that "If keeping the AF flag 0, software still can access the data from the corresponding address space ".

And this answer confuse me. Can anybody help to figure out that whether the CPU can access the data in the corresponding address when access flag is set to be 0?

Best Regards,

Shijun

Parents
  • 0 in reply to Martin Weidmann
    Hi Martin,

    Thanks for your reply.

    Assume that we are using the short descriptor format, and the hardware doesn't turn off the AF bit . Under this assumption, does the instruction access the memory region of the corresponding PTE? My understanding is the CPU cannot access the memory, only after the handler set the AF bit to be 1, then CPU can access the memory. Is my understanding right?

    Best Regards.
Reply
  • 0 in reply to Martin Weidmann
    Hi Martin,

    Thanks for your reply.

    Assume that we are using the short descriptor format, and the hardware doesn't turn off the AF bit . Under this assumption, does the instruction access the memory region of the corresponding PTE? My understanding is the CPU cannot access the memory, only after the handler set the AF bit to be 1, then CPU can access the memory. Is my understanding right?

    Best Regards.
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