Re: [Qemu-devel] [PATCH 08/10] intel-iommu: maintain per-device iova ranges

[Peter Xu <peterx@redhat.com>]

On Thu, May 03, 2018 at 05:22:03PM +0800, Jason Wang wrote:
> 
> 
> On 2018年05月03日 15:53, Peter Xu wrote:
> > On Thu, May 03, 2018 at 03:43:35PM +0800, Jason Wang wrote:
> > > 
> > > On 2018年05月03日 15:28, Peter Xu wrote:
> > > > On Thu, May 03, 2018 at 03:20:11PM +0800, Jason Wang wrote:
> > > > > On 2018年05月03日 14:04, Peter Xu wrote:
> > > > > > IMHO the guest can't really detect this, but it'll found that the
> > > > > > device is not working functionally if it's doing something like what
> > > > > > Jason has mentioned.
> > > > > > 
> > > > > > Actually now I have had an idea if we really want to live well even
> > > > > > with Jason's example: maybe we'll need to identify PSI/DSI.  For DSI,
> > > > > > we don't remap for mapped pages; for PSI, we unmap and remap the
> > > > > > mapped pages.  That'll complicate the stuff a bit, but it should
> > > > > > satisfy all the people.
> > > > > > 
> > > > > > Thanks,
> > > > > So it looks like there will be still unnecessary unamps.
> > > > Could I ask what do you mean by "unecessary unmaps"?
> > > It's for "for PSI, we unmap and remap the mapped pages". So for the first
> > > "unmap" how do you know it was really necessary without knowing the state of
> > > current shadow page table?
> > I don't.  Could I just unmap it anyway?  Say, now the guest _modified_
> > the PTE already.  Yes I think it's following the spec, but it is
> > really _unsafe_.  We can know that from what it has done already.
> > Then I really think a unmap+map would be good enough for us...  After
> > all that behavior can cause DMA error even on real hardwares.  It can
> > never tell.
> 
> I mean for following case:
> 
> 1) guest maps A1 (iova) to XXX
> 2) guest maps A2 (A1 + 4K) (iova) to YYY
> 3) guest maps A3 (A1 + 8K) (iova) to ZZZ
> 4) guest unmaps A2 and A2, for reducing the number of PSIs, it can
> invalidate A1 with a range of 2M
> 
> If this is allowed by spec, looks like A1 will be unmaped and mapped.

My follow-up patch won't survive with this one but the original patch
will work.

Jason and I discussed a bit on IRC on this matter.  Here's the
conclusion we got: for now we use my original patch (which solves
everything except PTE modifications).  We mark that modify-PTE problem
as TODO. Then at least we can have the nested device assignment work
well on known OSs first.

-- 
Peter Xu