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From: Jason Wang <jasowang@redhat.com>
To: Peter Xu <peterx@redhat.com>, David Gibson <david@gibson.dropbear.id.au>
Cc: qemu-devel@nongnu.org, mst@redhat.com, vkaplans@redhat.com,
	alex.williamson@redhat.com, wexu@redhat.com, pbonzini@redhat.com,
	cornelia.huck@de.ibm.com, dgibson@redhat.com
Subject: Re: [Qemu-devel] [PATCH 0/3] memory: add IOMMU notifier type
Date: Tue, 6 Sep 2016 14:26:44 +0800	[thread overview]
Message-ID: <782f9fec-498d-a121-b961-d9cb9a1c9473@redhat.com> (raw)
In-Reply-To: <20160906054927.GC21051@pxdev.xzpeter.org>



On 2016年09月06日 13:49, Peter Xu wrote:
> On Tue, Sep 06, 2016 at 03:06:17PM +1000, David Gibson wrote:
>> On Mon, Sep 05, 2016 at 03:21:18PM +0800, Peter Xu wrote:
>>> In the thread:
>>>
>>>    https://lists.gnu.org/archive/html/qemu-devel/2016-09/msg00254.html
>>>
>>> Alex proposed a way for vhost DMAR to be enabled without breaking
>>> existing protections on vIOMMU and device assignments. This series
>>> tried to implement the idea, by introducing a IOMMU notifier type for
>>> each IOMMU memory region.
>> Hrm, I'm pretty dubious about this concept, since it's basically just
>> an interim hack for an incomplete notifier implementation on x86.
>> What makes just fixing the notifier so difficult?
> Aviv is working on the full notifier support for that. It's been
> months since his last post though. If he cannot continue it (due to
> any reason), I can take it over. But for now, we may still need to
> wait for his patches to fully enable a complete notifier mechanism.

Yes, and the issue is:

- There's no way for current intel IOMMU code to be notified when guest 
map a page. So it's impossible for intel IOMMU to co-work with vfio now.
- A solution is caching mode (CM) support which requires a TLB 
invalidation even if it was a non-present to present changing, but this 
is still under development.
- VT-d spec requires: "Hardware implementations of this architecture 
must support operation corresponding to CM=0." So even if we have CM 
mode which can work with vfio, we still must support intel IOMMU with CM 
disabled. It was not only a spec requirement but also a performance 
consideration. (CM is usually slower)

So in conclusion, there's a mode for intel IOMMU that can't work with 
vfio at all. Fixing the notifier does not solve this since the notifier 
won't be able to be triggered.

>
> I don't know how POWER works to provide a complete notifier, and
> whether POWER can selectively enable the notified items... But for
> Intel VT-d, it provided two choices: by default, only cache
> invalidations are notified (even, we can disalbe cache invaliations),
> but if one want to have a complete notifier, just set the CM bit to 1.
> So I just think it'll be cool if we can support both cases. E.g., for
> vhost, it does not need to be notified with newly added entries, but
> only cache invalidations. IMHO we can't just force vhost to use a
> complete notifier while actually it only needs part of it.
>
> Thanks,
>
> -- peterx

  reply	other threads:[~2016-09-06  6:26 UTC|newest]

Thread overview: 28+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-09-05  7:21 [Qemu-devel] [PATCH 0/3] memory: add IOMMU notifier type Peter Xu
2016-09-05  7:21 ` [Qemu-devel] [PATCH 1/3] memory: add one flag for IOMMU notifier Peter Xu
2016-09-05  7:21 ` [Qemu-devel] [PATCH 2/3] memory: add iommu_notify_flag Peter Xu
2016-09-05  8:04   ` Paolo Bonzini
2016-09-05  8:38     ` Peter Xu
2016-09-05  9:56       ` Paolo Bonzini
2016-09-06  5:27         ` Peter Xu
2016-09-06  7:51           ` Paolo Bonzini
2016-09-06  8:17             ` Peter Xu
2016-09-06  8:19               ` Paolo Bonzini
2016-09-06 10:31                 ` Peter Xu
2016-09-07  5:44                   ` David Gibson
2016-09-07  6:34                     ` Peter Xu
2016-09-07  6:41                       ` David Gibson
2016-09-08  9:07                         ` Peter Xu
2016-09-12  1:26                           ` David Gibson
2016-09-12  5:13                             ` Peter Xu
2016-09-14  4:00                               ` David Gibson
2016-09-14  5:43                                 ` Peter Xu
2016-09-06  5:18       ` David Gibson
2016-09-06  5:55         ` Peter Xu
2016-09-06  5:12   ` David Gibson
2016-09-06  5:33     ` Peter Xu
2016-09-05  7:21 ` [Qemu-devel] [PATCH 3/3] intel_iommu: allow IOMMU_NONE typed notifiers Peter Xu
2016-09-06  5:06 ` [Qemu-devel] [PATCH 0/3] memory: add IOMMU notifier type David Gibson
2016-09-06  5:49   ` Peter Xu
2016-09-06  6:26     ` Jason Wang [this message]
2016-09-07  4:38       ` David Gibson

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