On Tue, Sep 2, 2025 at 4:20 AM Kalyazin, Nikita <[email protected]> wrote: > > [ based on kvm/next ] > > Implement guest_memfd allocation and population via the write syscall. > This is useful in non-CoCo use cases where the host can access guest > memory. Even though the same can also be achieved via userspace mapping > and memcpying from userspace, write provides a more performant option > because it does not need to set page tables and it does not cause a page > fault for every page like memcpy would. Note that memcpy cannot be > accelerated via MADV_POPULATE_WRITE as it is not supported by > guest_memfd and relies on GUP. > > Populating 512MiB of guest_memfd on a x86 machine: > - via memcpy: 436 ms > - via write: 202 ms (-54%)
Silly question: can you remind me why this speed-up is important? Also, I think we can get the same effect as MADV_POPULATE_WRITE just by making a second VMA for the memory file and reading the first byte of each page. Is that a viable strategy for your use case? Seems fine to me to allow write() for guest_memfd anyway. :)
