Re: [Qemu-devel] [PATCH RFC v3 14/14] intel_iommu: enable vfio devices

[Tian, Kevin]

> From: Peter Xu [mailto:address@hidden
> Sent: Wednesday, January 18, 2017 4:46 PM
> 
> On Wed, Jan 18, 2017 at 04:36:05PM +0800, Jason Wang wrote:
> >
> >
> > On 2017年01月18日 16:11, Peter Xu wrote:
> > >On Wed, Jan 18, 2017 at 11:10:53AM +0800, Jason Wang wrote:
> > >>
> > >>On 2017年01月17日 22:45, Peter Xu wrote:
> > >>>On Mon, Jan 16, 2017 at 05:54:55PM +0800, Jason Wang wrote:
> > >>>>On 2017年01月16日 17:18, Peter Xu wrote:
> > >>>>>>>  static void vtd_iotlb_page_invalidate(IntelIOMMUState *s, uint16_t
> domain_id,
> > >>>>>>>                                        hwaddr addr, uint8_t am)
> > >>>>>>>  {
> > >>>>>>>@@ -1222,6 +1251,7 @@ static void
> vtd_iotlb_page_invalidate(IntelIOMMUState *s, uint16_t domain_id,
> > >>>>>>>      info.addr = addr;
> > >>>>>>>      info.mask = ~((1 << am) - 1);
> > >>>>>>>      g_hash_table_foreach_remove(s->iotlb, vtd_hash_remove_by_page,
> &info);
> > >>>>>>>+    vtd_iotlb_page_invalidate_notify(s, domain_id, addr, am);
> > >>>>>>Is the case of GLOBAL or DSI flush missed, or we don't care it at all?
> > >>>>>IMHO we don't. For device assignment, since we are having CM=1 here,
> > >>>>>we should have explicit page invalidations even if guest sends
> > >>>>>global/domain invalidations.
> > >>>>>
> > >>>>>Thanks,
> > >>>>>
> > >>>>>-- peterx
> > >>>>Is this spec required?
> > >>>I think not. IMO the spec is very coarse grained on describing cache
> > >>>mode...
> > >>>
> > >>>>Btw, it looks to me that both DSI and GLOBAL are
> > >>>>indeed explicit flush.
> > >>>Actually when cache mode is on, it is unclear to me on how we should
> > >>>treat domain/global invalidations, at least from the spec (as
> > >>>mentioned earlier). My understanding is that they are not "explicit
> > >>>flushes", which IMHO should only mean page selective IOTLB
> > >>>invalidations.
> > >>Probably not, at least from the view of performance. DSI and global should
> > >>be more efficient in some cases.
> > >I agree with you that DSI/GLOBAL flushes are more efficient in some
> > >way. But IMHO that does not mean these invalidations are "explicit
> > >invalidations", and I suspect whether cache mode has to coop with it.
> >
> > Well, the spec does not forbid DSI/GLOBAL with CM and the driver codes had
> > used them for almost ten years. I can hardly believe it's wrong.
> 
> I think we have misunderstanding here. :)
> 
> I never thought we should not send DSI/GLOBAL invalidations with cache
> mode. I just think we should not do anything special even if we have
> cache mode on when we receive these signals.
> 
> In the spec, "explicit invalidation" is mentioned in the cache mode
> chapter:
> 
>     The Caching Mode (CM) field in Capability Register indicates if
>     the hardware implementation caches not-present or erroneous
>     translation-structure entries. When the CM field is reported as
>     Set, any software updates to any remapping structures (including
>     updates to not-present entries or present entries whose
>     programming resulted in translation faults) requires explicit
>     invalidation of the caches.
> 
> And I thought we were discussion about "what is explicit invalidation"
> mentioned above.

Check 6.5.3.1 Implicit Invalidation on Page Requests

        In addition to the explicit invalidation through invalidation commands 
        (see Section 6.5.1 and Section 6.5.2) or through deferred invalidation 
        messages (see Section 6.5.4), identified above, Page Requests from 
        endpoint devices invalidate entries in the IOTLBs and paging-structure 
        caches.

My impression is that above indirectly defines invalidation commands (
PSI/DSI/GLOBAL) as explicit invalidation, because they are explicitly
issued by driver. Then section 6.5.3.1 further describes implicit
invalidations caused by other VT-d operations.

I will check with VT-d spec owner to clarify.

> 
> >
> > >
> > >But here I should add one more thing besides PSI - context entry
> > >invalidation should be one of "the explicit invalidations" as well,
> > >which we need to handle just like PSI when cache mode is on.
> > >
> > >>>>Just have a quick go through on driver codes and find this something
> > >>>>interesting in intel_iommu_flush_iotlb_psi():
> > >>>>
> > >>>>...
> > >>>>     /*
> > >>>>      * Fallback to domain selective flush if no PSI support or the 
> > >>>> size is
> > >>>>      * too big.
> > >>>>      * PSI requires page size to be 2 ^ x, and the base address is 
> > >>>> naturally
> > >>>>      * aligned to the size
> > >>>>      */
> > >>>>     if (!cap_pgsel_inv(iommu->cap) || mask >
> cap_max_amask_val(iommu->cap))
> > >>>>         iommu->flush.flush_iotlb(iommu, did, 0, 0,
> > >>>>                         DMA_TLB_DSI_FLUSH);
> > >>>>     else
> > >>>>         iommu->flush.flush_iotlb(iommu, did, addr | ih, mask,
> > >>>>                         DMA_TLB_PSI_FLUSH);
> > >>>>...
> > >>>I think this is interesting... and I doubt its correctness while with
> > >>>cache mode enabled.
> > >>>
> > >>>If so (sending domain invalidation instead of a big range of page
> > >>>invalidations), how should we capture which pages are unmapped in
> > >>>emulated IOMMU?
> > >>We don't need to track individual pages here, since all pages for a 
> > >>specific
> > >>domain were unmapped I believe?
> > >IMHO this might not be the correct behavior.
> > >
> > >If we receive one domain specific invalidation, I agree that we should
> > >invalidate the IOTLB cache for all the devices inside the domain.
> > >However, when cache mode is on, we should be depending on the PSIs to
> > >unmap each page (unless we want to unmap the whole address space, in
> > >this case it's very possible that the guest is just unmapping a range,
> > >not the entire space). If we convert several PSIs into one big DSI,
> > >IMHO we will leave those pages mapped/unmapped while we should
> > >unmap/map them.
> >
> > Confused, do you have an example for this? (I fail to understand why DSI
> > can't work, at least implementation can convert DSI to several PSIs
> > internally).
> 
> That's how I understand it. It might be wrong. Btw, could you
> elaborate a bit on how can we convert a DSI into several PSIs?
> 
> Thanks,

If my understanding above is correct, there is nothing wrong with 
above IOMMU driver code - actually it makes sense on bare metal
when CM is disabled.

But yes, DSI/GLOBAL is far less efficient than PSI when CM is enabled.
We rely on cache invalidations to indirectly capture remapping structure 
change. PSI provides accurate info, while DSI/GLOBAL doesn't. To 
emulate correct behavior of DSI/GLOBAL, we have to pretend that
the whole address space (iova=0, size=agaw) needs to be unmapped
(for GLOBAL it further means multiple address spaces)

Though not efficient, it doesn't mean it's wrong since guest driver
follows spec. We can ask for linux IOMMU driver change (CC Ashok)
to not use above optimization when cache mode is enabled, but 
anyway we need emulate correct DSI/GLOBAL behavior to follow
spec, because:

- even when driver fix is in place, old version still has this logic;

- there is still scenario where guest IOMMU driver does want to
invalidate the whole address space, e.g. when changing context
entry. Asking guest driver to use PSI for such purpose is another
bad thing.

Thanks
Kevin