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

[Jordan Justen]

On Sun, Mar 4, 2012 at 02:46, Avi Kivity <address@hidden> wrote:
> On 03/04/2012 12:38 PM, Gleb Natapov wrote:
>> On Sun, Mar 04, 2012 at 12:36:20PM +0200, Avi Kivity wrote:
>> > On 03/04/2012 10:12 AM, Gleb Natapov wrote:
>> > > On Sat, Mar 03, 2012 at 10:56:02PM -0800, Jordan Justen wrote:
>> > > > 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.
>> > > Do you want to execute code from a flash while it traps on read/exec?
>> > > KVM emulator can't do that.
>> >
>> > No, he wants read/exec to go to RAM, and writes to be handled as mmio.
>> > This should work.
>> >
>> That's mode 1 from above and will obviously work. I am asking about mode
>> 2 from above.
>>
>
> Mode 2 is pure mmio.  You're right that kvm won't run code from that.
>
> Jordan, what's the use for mode 2?  Is it just used during programming?

Yes.  It is the programming state for the flash, and no execution
would occur from the region while it is in that state.  At least for
the flash that I'm interested in supporting (qemu:hw/pflash_cfi01.c),
it would not be possible to execute from the flash in this mode
because the entire region will no longer read-back the flash contents.

-Jordan