Re: [RISC-V] [tech-virt-mem] Help needed on physical address issues


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On Tue, Aug 9, 2022 at 3:24 AM Ke Chai <ck.rickytino@...> wrote:
To whom it may concern,

Hello! I am currently working on a RV64 CPU featuring Sv48 and Sv39, and came up with several questions that none of the existing documents have clearly specified: 

1. The physical address width in my RV64 implementation is 50 bits. If a 56-bit address is generated (e.g. from a leaf PTE's PPN concatenates an address offset, or from a physical address translated directly from a virtual address under M mode) with non-zero at the high bits (i.e. bits 55-50), should I just ignore the high bits, or report an access fault exception (assuming this is recognized as an vacant address space during a PMA check)?

2. Is there any way the M-mode software can know how many physical address bits are implemented on this implementation?

3. I saw a sentence on the RISC-V privileged architecture document at the end of section 4.3.2:
"The algorithm does not admit the possibility of ignoring high-order PPN bits for implementations with narrower physical addresses". What does "does not admit the possibility" supposed to mean? What action would be taken by hardware if the high-order PPN bits are not zero?

Really looking forward to someone who can give me some clarification on this! 

Thank you so much,
Ricky Chai

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