[Qemu-devel] KVM/QEMU: Support executing from flash device

[Jordan Justen]

On Tue, Oct 25, 2011 at 00:47, Avi Kivity <address@hidden> wrote:
> The core issue that kvm (the kernel part) supports two styles of memory:
> read/write RAM, and read/write MMIO.  ROM wants writes to be ignored,
> and rom/device wants reads serviced from memory and writes serviced by
> userspace (as MMIO).
>
> It should not be too hard to patch kvm to support these additional two
> styles.  The entry point is the KVM_SET_USER_MEMORY_REGION ioctl to
> define the new attributes for the region, and kvm_mmu_page_fault() to
> map these pages as read-only and emulate writes (for ROM/device regions).

Additional context for CC'd kvm-devel: I would like to support
executing from an emulated flash memory on kvm.  The flash memory
would be a CFI (Firmware Hub-like) device which is programmed via MMIO
at the same addresses in memory where the execution would occur.

What is needed is:
1. The ability to set a region to allow read/exec,
   but trap to qemu on writes.
2. qemu should then be able to set the region
   to trap on reads/exec/writes.
3. qemu should be able to then restore the region
   to the initial state.

Here is my plan:
1. Add KVM_CAP_REGION_WRITE_TRAP
2. Add kvm_memory_region::flags
   KVM_MEMSLOT_TRAP_WRITES
3. Update page table creation to trap on writes when
   KVM_MEMSLOT_TRAP_WRITES is set

Is this plan heading in the right direction?

Regarding emulating ROM (as you mentioned above): I think that it can
trap to qemu and QEMU can ignore it.  This would be a performance hit,
but I don't think it is expected that lots of writes to a ROM will
occur.  Do you agree?

-Jordan