Miri's hardcoded PAGE_SIZE is sometimes inaccurate

[thomcc]

Miri always claims the page size is 4k, but on some targets this is not the case. Programs generally should query it at runtime, but a lot of code just hard-codes 4k (like miri), which is not right sometimes.

Plausibly this is an argument for it being configurable, but I think that's likely overkill (and deciding what values are allowable seems tricky). Thankfully, the targets I know of where this varies are seem to be statically fixed in practice, so Miri should just return the accurate-in-practice value. Specifically:

• On aarch64-apple-darwin the page size seems to be 16k (apple made a point about this changing in the move to M1). (Unsure if miri supports it, but this is true for aarch64-apple-ios too, and perhaps all aarch64-apple-*).
• The web assembly spec defines it as 64k (kinda wild, but so it goes): https://webassembly.github.io/spec/core/exec/runtime.html#memory-instances, and WASI seems to follow this.

I don't know of other targets which happen to have different values, and have looked around a bit (according to https://linux.die.net/man/2/getpagesize, on sun4 is a real example where it depended on the specific machine rather than the target, but Rust probably doesn't support it).

Anyway, currently it doesn't matter much, but if #2520 lands (CC @saethlin) it would matter more. It might make sense to do as part of that PR, but this is certainly not necessary, so I filed it as a separate thing rather than asking on that PR.

[RalfJung]

I don't know of other targets which happen to have different values, and have looked around a bit (according to https://linux.die.net/man/2/getpagesize, on sun4 is a real example where it depended on the specific machine rather than the target, but Rust probably doesn't support it).

Seems okay for Miri to define it as 4K then as long as that is a legal option for the target. ;)

But indeed if other targets define it to definitely be something else, Miri should respect that. A separate PR that replaces the constant by a function which takes into account the target would be appreciated.

[bjorn3]

On aarch64-apple-darwin the page size seems to be 16k (apple made a point about this changing in the move to M1). (Unsure if miri supports it, but this is true for aarch64-apple-ios too, and perhaps all aarch64-apple-*).

Even worse aarch64-unknown-linux-gnu on M1 is 16k too (as mmio has to be mapped with 16k pages and linux doesn't support mixing 4k and 16k pages), but on other systems it is 4k.