Re: [Qemu-devel] [PATCH 2/4] Add support for execution from ROMs in IO device mode

[Jan Kiszka <jan.kiszka@web.de>]

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Jan Kiszka wrote:
> Jamie Lokier wrote:
>> Jan Kiszka wrote:
>>> While IO_MEM_ROMD marks an I/O memory region as "read/execute from RAM,
>>> but write to I/O handler", there is no flag indicating that an I/O
>>> region which is fully managed by I/O handlers can still be hosting
>>> executable code. One use case for this are flash device models that
>>> switch to I/O mode during reprogramming. Not all reprogramming states
>>> modify to read data, thus practically allow to continue execution.
>>> Moreover, we need to avoid switching the modes too frequently for
>>> performance reasons which requires fetching opcodes while still in I/O
>>> device mode.
>> I like this change.
>>
>> Does "fetching opcodes while still in I/O device mode" fetch opcodes
>> from the RAM backing, or via the I/O read handlers?
> 
> Via the latter. This is most "correct" in that broken guests that jump
> to the ROM while it's in some critical write cycle will properly crash.
> 
>> If the latter, I'm wondering how KVM would cope with that.
> 
> I think it won't work without extending KVM to fetch - as a last resort
> - also from I/O devices. Or we give up the "correctness" and keep the
> RAM-base KVM slot for IO_MEM_EXEC regions. That should be solvable in a
> transparent way in kvm_set_phys_mem.

Which, of course, would break CFI reads etc. for KVM guest. So this
feature actually requires additional support by KVM to fetch code from
I/O devices.

Jan


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