Attempting to add NX bit

[DimaThenekov]

Thanks to the comment #845 (comment) I tried adding the NX-bit, but I ran into the definition of read for execution and read for data. Can you give any implementation tips?

When starting an OS that requires NX-bit (Windows 8/10), I see a message in the logs about enabling the NX bit, but the result is the same as before.

1. I understand that this is a difficult task, but the number of function calls to read from memory is too large. Can this be simplified?
2. Am I doing the right thing by storing the nx flag in tlb?
3. I also don't quite understand how the implementation of the nx bit can improve OS support, since this flag is only needed for security.
4. I don't know how to test this. I couldn't run any kvm tests other than realmode.flat.

[copy]

I understand that this is a difficult task, but the number of function calls to read from memory is too large. Can this be simplified?

Sure, I'm open to suggestions. A good starting point is the read_imm family of functions, which is by far the most common case of reading instructions.

Am I doing the right thing by storing the nx flag in tlb?

Yes, I believe that's the right approach.

I also don't quite understand how the implementation of the nx bit can improve OS support, since this flag is only needed for security.

Supposedly Windows 8 doesn't work without NX support unless you use workarounds. I haven't tested it. And it's a pre-req for 64-bit support.

I don't know how to test this. I couldn't run any kvm tests other than realmode.flat.

At the very least:

• Boot some OSes and check that they detect and apply the NX bit. This can be added to tests/full/run.js
• Write some unit tests that set up a page tables with NX protection, jump into them and observe that the page fault is being hit. This could be added to tests/nasm or tests/qemu or tests/jit-paging (this one could be forked) or kvm-unit-test.