Firecracker and Crosvm Common Crates - WIP

firecracker_crosvm_crates.md

Firecracker and Crosvm Common Crates

The purpose of this gist is to get a discussion started about what the Firecracker team can contribute to crosvm.

This comparison is based on the following commits:

• crosvm commit 7df5a0ef1a23542b0cc7db7797b8b287eaa05699
• Firecracker commit d6861f91acc13868cac4cde94cbce21d9cb5e833

This comparison only touches the common crates of Firecracker and crosvm.

Kernel-related crates

In Firecracker we unified the kernel_loader and kernel_cmdline crates in one crate with two modules:

• loader - corresponds to kernel_loader in crosvm
• cmdline - corresponds to kernel_cmdline in crosvm

Firecracker changes to cmdline

• added derive clone to the Cmdline structure
• in the constructor, we use the capacity to initialize the string as advertised by the doc string.

Firecracker changes to kernel loader:

• Removed unnecessary defines from elf.rs. This file was originally generated with bindgen from linux “include/uapi/linux/elf.h” file.
• Added an extra sanity check for e_entry, which is required to be < the high memory start.
• The load_kernel function returns the entry address of the kernel.

Crosvm changes:

• load_kernel function has a new parameter in the signature: kernel_start. The parameter is used for computing the guest memory offset.

The unit tests are also changed a bit.

Potential changes to contribute to crosvm

• kernel_loader: e_entry sanity check
• kernel_loader: stripped down version of the elf.rs file
• kernel_cmdline: initialize the cmdline string with the capacity specified in the function signature.

Memory-related

In Firecracker we moved all the memory related utilities in the memory_model crate. Initially this crate was the same as the data_model crate from crosvm.

We moved the following files from sys_util to memory_model: guest_address.rs, guest_memory.rs, mmap.rs.

Firecracker changes:

guest_address.rs

• The GuestAddress is represented as usize instead of u64.
• Added more unit tests

guest_memory.rs

• Added map and fold function for memory regions. This is a helper function for counting the guest dirty pages.
• Added immutable version of the with_regions function.

mmap.rs

• Use overflowing_add() to validate requested address range and avoid panicking at runtime.

Crosvm Changes:

guest_memory.rs

• Function for getting the memory size (we should also do this instead of making the MemoryRegion a public structure).
• Function for madvising an adress range
• Implementation for Volatile Memory.

mmap.rs

• When initializing the memory, use madvise MADV_DONTDUMP, which specifies that the memory regions should be excluded from a core dump.
• split from_fd in 2 functions: from_fd and from_fd_offset.
• function for removing memory region (using madvise).
• volatile memory support.

Potential changes to contribute to crosvm

• Overflow add in mmap.rs
• Others?.

KVM-related crates

In Firecracker all generated files are in _gen crates. KVM bindings are generated from the kernel version 4.13. In Crosvm, the autogenerated files are in _sys crates. The implementation of the kvm wrappers seems to have divereged, so I am just going to highlight some things that might be useful to contribute back.

Potential changes to contribute to crosvm:

• Added get_api_version
• Updated check_extensionto return true for KVM_CHECK_EXTENSION >=1 as per KVM documentation (before it was == 1).
• Others?