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

From: Jason Wang
Subject: Re: [Qemu-devel] [PATCH RFC v3 14/14] intel_iommu: enable vfio devices
Date: Thu, 19 Jan 2017 15:02:37 +0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.5.1

On 2017年01月19日 14:44, Liu, Yi L wrote:
-----Original Message-----
From: Qemu-devel [mailto:address@hidden
On Behalf Of Tian, Kevin
Sent: Wednesday, January 18, 2017 5:39 PM
To: Peter Xu <address@hidden>; Jason Wang <address@hidden>
Cc: Lan, Tianyu <address@hidden>; Raj, Ashok <address@hidden>;
address@hidden; address@hidden; address@hidden; qemu-
address@hidden; address@hidden
Subject: Re: [Qemu-devel] [PATCH RFC v3 14/14] intel_iommu: enable vfio

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,
                                        hwaddr addr, uint8_t
@@ -1222,6 +1251,7 @@ static void
vtd_iotlb_page_invalidate(IntelIOMMUState *s, uint16_t domain_id,
      info.addr = addr;
      info.mask = ~((1 << am) - 1);
+    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.


-- 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
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

     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 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

My impression is that above indirectly defines invalidation commands (
PSI/DSI/GLOBAL) as explicit invalidation, because they are explicitly issued by
driver. Then section 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
      * aligned to the size
     if (!cap_pgsel_inv(iommu->cap) || mask >
         iommu->flush.flush_iotlb(iommu, did, 0, 0,
         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?

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

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 
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.
Hi Kevin/Peter/Jason,

I agree we should think DSI/GLOBAL. Herby, I guess there may be a chance to 
DSI/GLOBAL flush if the following assumption is correct.

It seems like that all DSI/GLOBAL flush would always be after a context entry 

Well it looks like at least for DSI, flush could happen when the size is too big?

With this assumption, I remember Peter added memory_replay in context 
This memory_replay would walk guest second-level page table and do map. So the
second-level page table in host should be able to get the latest mapping info. 
IOMMU driver would issue an DSI/GLOBAL flush after changing context. Since the
mapping info has updated in host, then there is no need to deal this DSI/GLOBAL 

So gentlemen, pls help judge if the assumption is correct. If it is correct, 
then Peter's patch
may just work without special process against DSI/GLOBAL flush.
Yi L

Even if this may be the usual case, I think we'd better not make the codes depends on (usual) guest behaviors.


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