Allow mapping MAP_PRIVATE memory to file

[acatangiu]

The only provided interfaces right now force MAP_SHARED on all file mappings.

[sandreim]

Current solution is this:

 fn restore(file: &File, state: &GuestMemoryState) -> std::result::Result<Self, Error> {
        let mmap_regions = state
            .regions
            .iter()
            .map(|region| {
                let file_clone = file.try_clone().expect("Cannot clone memory file.");
                MmapRegion::build(
                    Some(FileOffset::new(file_clone, region.offset)),
                    region.size,
                    libc::PROT_READ | libc::PROT_WRITE,
                    libc::MAP_NORESERVE | libc::MAP_PRIVATE,
                )
                .map(|r| {
                    GuestRegionMmap::new(r, GuestAddress(region.base_address))
                        .expect("Cannot create guest mmap region.")
                })
                .expect("Cannot create mmap region.")
            })
            .collect();

      Ok(Self::from_regions(mmap_regions).map_err(Error::CreateMemory)?)
}

[mikkorantalainen]

Is there any plans to support this natively? I was surprised to find that memmap::MmapOptions always uses MAP_SHARED.

[andreeaflorescu]

Ping @bonzini @jiangliu @alexandruag