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

[Jan Kiszka]

On 2016-03-01 21:11, Michael S. Tsirkin wrote:
> On Tue, Mar 01, 2016 at 03:00:09PM +0100, Jan Kiszka wrote:
>> On 2016-03-01 14:48, Jan Kiszka wrote:
>>> There is likely no way around write-protecting the IOMMU page tables (in
>>> KVM mode) once we evaluated and cached them somewhere.
>>
>> I mean, when in kvm mode AND having something that caches enabled, of
>> course.
> 
> Just write-protecting won't be enough either, since
> the moment you remove the protection, all bets are off,
> and if you don't, guest will start from the same point
> when you re-enter and fault again.

We would not remove protection as long as the entry is in use by the
IOMMU. There should be no difference from shadow MMU logic here: trap
and emulate the write.

> 
> 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

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