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Re: [Qemu-devel] [PATCH 08/10] intel-iommu: maintain per-device iova ran
From: |
Peter Xu |
Subject: |
Re: [Qemu-devel] [PATCH 08/10] intel-iommu: maintain per-device iova ranges |
Date: |
Thu, 3 May 2018 17:53:59 +0800 |
User-agent: |
Mutt/1.9.3 (2018-01-21) |
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