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Re: [Qemu-devel] [PATCH 2/3] memory: add iommu_notify_flag
From: |
Peter Xu |
Subject: |
Re: [Qemu-devel] [PATCH 2/3] memory: add iommu_notify_flag |
Date: |
Thu, 8 Sep 2016 17:07:32 +0800 |
User-agent: |
Mutt/1.5.24 (2015-08-30) |
On Wed, Sep 07, 2016 at 04:41:54PM +1000, David Gibson wrote:
> On Wed, Sep 07, 2016 at 02:34:19PM +0800, Peter Xu wrote:
> > On Wed, Sep 07, 2016 at 03:44:19PM +1000, David Gibson wrote:
> > > > For "CHANGE", it sounds like a unmap() + a map(). However I'd say
> > > > "ADDITION" is nowhere better...
> > >
> > > Right.. this brings up a good point.
> > >
> > > Changing a mapping (i.e. overwriting an existing mapping with a
> > > different one) would also need notification, even on x86, no? Since
> > > it implicitly invalidates the previous mapping.
> > >
> > > I'm guessing the guest will avoid this by always unmapping before it
> > > maps. We still need to consider this possibility when designing the
> > > notifier interface though.
> > >
> > > It seems the real notification triggers here are:
> > > * map - something is mapped which previously wasn't
> > > * unmap - something is no longer mapped which was before
> > >
> > > Note that whether the second needs to be triggered depends on the
> > > *previous* state of that IOBA range, *not* on the permissions of the
> > > new mapping (if any).
> > >
> > > A "change" - replacing one mapping with another should count as both a
> > > "map" and "unmap" event.
> >
> > Yeah... For MAP/UNMAP, it is strange in another way: e.g. for vhost,
> > it doesn't care about map/unmap, it cares about invalidated cache.
>
> I think caring about invalidated cache *is* caring about unmap. It
> doesn't matter whether the new mapping is something or nothing - if
> the old mapping is no longer valid, you need to invalidate the cache,
> yes?
Yes, I think these two are exactly the same in implementation (vhost
needs UNMAP events of course). So that's why I called it "a naming
issue". :)
>
> > So
> > IIUC this is a question about "naming" but not the implementations...
> > I suppose it is really a matter of taste, and both work for me (either
> > INVALIDATION/CHANGE or UNMAP/MAP).
>
> No.. it is a question of implementation. My point is that I don't
> think the new permission is sufficient information to let you know if
> a notification is necessary. You need to know if there was an
> existing mapping at that IOBA.
My understanding is that we don't need to know that. Because IIUC
there are only map_page() and unmap_page() in guest IOMMU driver
(please check dma_map_ops in kernel). There is no chance for anyone to
"change" the content of the mapping, unless it calls unmap_page() then
with a map_page(). In that case, we'll have two IOTLB invalidation
requests.
Please kindly correct me if I am wrong.
Thanks,
-- peterx
- Re: [Qemu-devel] [PATCH 2/3] memory: add iommu_notify_flag, (continued)
- Re: [Qemu-devel] [PATCH 2/3] memory: add iommu_notify_flag, Peter Xu, 2016/09/05
- Re: [Qemu-devel] [PATCH 2/3] memory: add iommu_notify_flag, Paolo Bonzini, 2016/09/05
- Re: [Qemu-devel] [PATCH 2/3] memory: add iommu_notify_flag, Peter Xu, 2016/09/06
- Re: [Qemu-devel] [PATCH 2/3] memory: add iommu_notify_flag, Paolo Bonzini, 2016/09/06
- Re: [Qemu-devel] [PATCH 2/3] memory: add iommu_notify_flag, Peter Xu, 2016/09/06
- Re: [Qemu-devel] [PATCH 2/3] memory: add iommu_notify_flag, Paolo Bonzini, 2016/09/06
- Re: [Qemu-devel] [PATCH 2/3] memory: add iommu_notify_flag, Peter Xu, 2016/09/06
- Re: [Qemu-devel] [PATCH 2/3] memory: add iommu_notify_flag, David Gibson, 2016/09/07
- Re: [Qemu-devel] [PATCH 2/3] memory: add iommu_notify_flag, Peter Xu, 2016/09/07
- Re: [Qemu-devel] [PATCH 2/3] memory: add iommu_notify_flag, David Gibson, 2016/09/07
- Re: [Qemu-devel] [PATCH 2/3] memory: add iommu_notify_flag,
Peter Xu <=
- Re: [Qemu-devel] [PATCH 2/3] memory: add iommu_notify_flag, David Gibson, 2016/09/11
- Re: [Qemu-devel] [PATCH 2/3] memory: add iommu_notify_flag, Peter Xu, 2016/09/12
- Re: [Qemu-devel] [PATCH 2/3] memory: add iommu_notify_flag, David Gibson, 2016/09/14
- Re: [Qemu-devel] [PATCH 2/3] memory: add iommu_notify_flag, Peter Xu, 2016/09/14
- Re: [Qemu-devel] [PATCH 2/3] memory: add iommu_notify_flag, David Gibson, 2016/09/06
- Re: [Qemu-devel] [PATCH 2/3] memory: add iommu_notify_flag, Peter Xu, 2016/09/06
Re: [Qemu-devel] [PATCH 2/3] memory: add iommu_notify_flag, David Gibson, 2016/09/06
[Qemu-devel] [PATCH 3/3] intel_iommu: allow IOMMU_NONE typed notifiers, Peter Xu, 2016/09/05
[Qemu-devel] [PATCH 1/3] memory: add one flag for IOMMU notifier, Peter Xu, 2016/09/05