io: make Read/WriteVolatile implementations optional

[bonzini]

Summary of the PR

Some projects may not want I/O to go directly to libc, for example to avoid any kind of blocking I/O. Make the file descriptor implementations of ReadVolatile and WriteVolatile optional.

The PR is on top of #310.

Requirements

Before submitting your PR, please make sure you addressed the following
requirements:

• All commits in this PR have Signed-Off-By trailers (with
  git commit -s), and the commit message has max 60 characters for the
  summary and max 75 characters for each description line.
• All added/changed functionality has a corresponding unit/integration
  test.
• All added/changed public-facing functionality has entries in the "Upcoming
  Release" section of CHANGELOG.md (if no such section exists, please create one).
• Any newly added unsafe code is properly documented.

[roypat]

Btw Paolo, while I got your attention here in rust-vmm land, any chance you have a second to look at #307? :o

[stefano-garzarella]

@bonzini can you check tests when only backend-mmap or xen is enabled?

I had something like that (with xen looks similar):

$ cargo test --no-default-features --features=backend-mmap
   Compiling vm-memory v0.16.1 (/home/stefano/repos/vm-memory)
error[E0277]: the trait bound `std::fs::File: io::ReadVolatile` is not satisfied
    --> src/mmap.rs:1165:47
     |
1165 |             gm.read_exact_volatile_from(addr, &mut file, mem::size_of::<u32>())
     |                ------------------------       ^^^^^^^^^ the trait `io::ReadVolatile` is not implemented for `std::fs::File`
     |                |
     |                required by a bound introduced by this call
     |
     = help: the following other types implement trait `io::ReadVolatile`:
               &[u8]
               std::io::Cursor<T>
note: required by a bound in `guest_memory::GuestMemory::read_exact_volatile_from`
    --> src/guest_memory.rs:739:12
     |
732  |     fn read_exact_volatile_from<F>(
     |        ------------------------ required by a bound in this associated function
...
739  |         F: ReadVolatile,
     |            ^^^^^^^^^^^^ required by this bound in `GuestMemory::read_exact_volatile_from`

For more information about this error, try `rustc --explain E0277`.
error: could not compile `vm-memory` (lib test) due to 1 previous error

This reminds me that we should work on rust-vmm/rust-vmm-ci#152
I'll try to allocate some time after FOSDEM.

[bonzini]

All combinations (tested with cargo hack --feature-powerset build --tests, while test --all is in progress) work now.

[stefano-garzarella]

All combinations (tested with cargo hack --feature-powerset build --tests, while test --all is in progress) work now.

Thanks for checking!

[stefano-garzarella]

@bonzini I think there is another failure in some examples which are tested only when running the tests (so maybe you already discovered running the all tests, but I'm reporting in any case):

$ git describe              
v0.11.0-101-ga132296

$ cargo test --no-default-features --features backend-mmap
...
failures:

---- src/guest_memory.rs - guest_memory::GuestMemory::read_volatile_from (line 652) stdout ----
error[E0277]: the trait bound `File: ReadVolatile` is not satisfied
   --> src/guest_memory.rs:672:29
    |
23  | gm.read_volatile_from(addr, &mut file, 128)
    |    ------------------       ^^^^^^^^^ the trait `ReadVolatile` is not implemented for `File`
    |    |
    |    required by a bound introduced by this call
    |
    = help: the following other types implement trait `ReadVolatile`:
              &[u8]
              std::io::Cursor<T>
note: required by a bound in `read_volatile_from`
   --> /home/stefano/repos/vm-memory/src/guest_memory.rs:682:12
    |
680 |     fn read_volatile_from<F>(&self, addr: GuestAddress, src: &mut F, count: usize) -> ...
    |        ------------------ required by a bound in this associated function
681 |     where
682 |         F: ReadVolatile,
    |            ^^^^^^^^^^^^ required by this bound in `GuestMemory::read_volatile_from`

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0277`.
Couldn't compile the test.

failures:
    src/guest_memory.rs - guest_memory::GuestMemory::read_volatile_from (line 652)

test result: FAILED. 42 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.97s

[bonzini]

I forgot to push that one, sorry.