ARM64: Adding write-protect bit for Userfaultfd

[Catalin Marinas]

On Fri, Apr 28, 2023 at 01:35:44PM -0400, Mohamed Husain Noor Mohamed wrote:
> I am Mohamed Husain. I am a graduate student, working on a research
> project using a userfaultfd for distributed shared memory.
> We are trying to use Write-Protect mode in ARM64 but based on the
> kernel commits we see the support only exists for the x86 kernel.
> https://lwn.net/Articles/777258/
> 
> I am trying to add the write-protect support, so I am looking for the
> unused bits in the PTE. Do you guys have any suggestions on the bits I
> could use, or does it require hardware support?

Unfortunately, we are pretty short on bits. The architecture only gives
us bits 55 to 58 and they are all used. We could move PTE_PROT_NONE to
another position (e.g. 60) since this is only used when !PTE_VALID and
therefore doesn't affect the actual page attributes and free up a bit.

Alternatively, we could hijack bits 59-62 but there may be out of tree
patches making use of the PBHA imp def feature (AFAICT disabled on the
mainline kernel). Well, I guess one could make the userfaultfd wp
feature conditional.

Cc'ing Anshuman as well, I think he looked at soft-dirty ptes for arm64
before for CRIU live migration and we concluded that userfaultfd was
better but I didn't realise that the write-protect mechanism needs its
own PTE bit as well.

-- 
Catalin