Add sfence.vma in the disable_pmp function

[lz-bro]

Change-Id: Ief659c0e68618f5b08a1eee98bc7df92817b8b51

[aswaterman]

SFENCE.VMA is illegal on implementations without S-mode. I think this should be conditionalized.

[lz-bro]

SFENCE.VMA is illegal on implementations without S-mode. I think this should be conditionalized.

PMP changes require an SFENCE.VMA on any hart that implements page-based virtual memory, even if VM is not currently enabled. If the implementation without S-mode, does this mean that PMP registers are optional？

[aswaterman]

As you said, "any hart that implements page-based virtual memory" needs to execute SFENCE.VMA. If S-mode is not implemented, then the hart doesn't implement page-based virtual memory. So there's no contradiction here.

But I did misspeak earlier. It is true that SFENCE.VMA is illegal on implementations without S-mode. But it is also illegal on implementations with S-mode but without virtual memory. So, it needs to be conditionalized on "is S-mode implemented" and "is page-based virtual memory implemented". You can determine the latter property by checking whether satp.MODE is hardwired to 0.

But note that writing satp is illegal unless S-mode is implemented. So you should first check if S-mode is implemented, then check if satp.MODE is writable, and only then should you execute SFENCE.VMA.

[lz-bro]

As you said, "any hart that implements page-based virtual memory" needs to execute SFENCE.VMA. If S-mode is not implemented, then the hart doesn't implement page-based virtual memory. So there's no contradiction here.

But I did misspeak earlier. It is true that SFENCE.VMA is illegal on implementations without S-mode. But it is also illegal on implementations with S-mode but without virtual memory. So, it needs to be conditionalized on "is S-mode implemented" and "is page-based virtual memory implemented". You can determine the latter property by checking whether satp.MODE is hardwired to 0.

But note that writing satp is illegal unless S-mode is implemented. So you should first check if S-mode is implemented, then check if satp.MODE is writable, and only then should you execute SFENCE.VMA.

Thank you for your explanation，I've introduced an implements_page_virtual_memory property for the target. But, There is a pipeline failure in Build Spike.

[aswaterman]

I think we need to do something similar to the following. Can you give it a try and see if you can make it work? https://github.com/riscv-software-src/riscv-isa-sim/pull/1584/files

[lz-bro]

I think we need to do something similar to the following. Can you give it a try and see if you can make it work? https://github.com/riscv-software-src/riscv-isa-sim/pull/1584/files

Yes, It works now.

[lz-bro]

@aswaterman Would you kindly review and merge it.