Re: [Qemu-devel] [V6 0/4] AMD IOMMU

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2016-03-01 21:39, Michael S. Tsirkin wrote:
> On Tue, Mar 01, 2016 at 09:17:58PM +0100, Jan Kiszka wrote:
>> On 2016-03-01 21:11, Michael S. Tsirkin wrote:
>>>
>>> What this seems to call for is a new kind of protection
>>> where yes PTE is write protected, but instead of
>>> making PTE writeable (or killing guest)
>>> KVM handles it as an MMIO: emulates the write and then skips the instruction.
>>>
>>> Emulation can be in kernel, just writing into guest memory
>>> on behalf of the guest - with some kind of notifier
>>> to flush the vfio cache - or instead it can exit to userspace
>>> and have QEMU handle it like MMIO and write into guest memory.
>>
>> Exactly, but that's nothing new, is it? It's "just" slow, like other
>> shadow MMUs.
>>
>> Jan
> 
> Well AFAIK KVM does not have such an option ATM: MMIO causes exits for
> reads and writes.  We want MMIO exits for writes but not reads.
> I agree it should be easy to implement.

We have read-only memory slot support for quite a while. It's used to
support execute (and read) from emulated ROM devices.

Jan

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