Re: KVM/QEMU: Support executing from flash device

[Gleb Natapov <gleb@redhat.com>]

On Sat, Mar 03, 2012 at 10:56:02PM -0800, Jordan Justen wrote:
> On Tue, Oct 25, 2011 at 00:47, Avi Kivity <avi@redhat.com> 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.
Do you want to execute code from a flash while it traps on read/exec?
KVM emulator can't do that.

> 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

--
			Gleb.