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 -- Siemens AG, Corporate Technology, CT RDA ITP SES-DE Corporate Competence Center Embedded Linux
