Re: [RFC v2 1/1] memory: Delete assertion in memory_region_unregister_iommu_notifier

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Jun 30, 2020 at 04:29:19PM +0800, Jason Wang wrote:
> 
> On 2020/6/30 上午10:41, Jason Wang wrote:
> > 
> > On 2020/6/29 下午9:34, Peter Xu wrote:
> > > On Mon, Jun 29, 2020 at 01:51:47PM +0800, Jason Wang wrote:
> > > > On 2020/6/28 下午10:47, Peter Xu wrote:
> > > > > On Sun, Jun 28, 2020 at 03:03:41PM +0800, Jason Wang wrote:
> > > > > > On 2020/6/27 上午5:29, Peter Xu wrote:
> > > > > > > Hi, Eugenio,
> > > > > > > 
> > > > > > > (CCing Eric, Yan and Michael too)
> > > > > > > 
> > > > > > > On Fri, Jun 26, 2020 at 08:41:22AM +0200, Eugenio Pérez wrote:
> > > > > > > > diff --git a/memory.c b/memory.c
> > > > > > > > index 2f15a4b250..7f789710d2 100644
> > > > > > > > --- a/memory.c
> > > > > > > > +++ b/memory.c
> > > > > > > > @@ -1915,8 +1915,6 @@ void
> > > > > > > > memory_region_notify_one(IOMMUNotifier
> > > > > > > > *notifier,
> > > > > > > >             return;
> > > > > > > >         }
> > > > > > > > -    assert(entry->iova >= notifier->start &&
> > > > > > > > entry_end <= notifier->end);
> > > > > > > I can understand removing the assertion should solve
> > > > > > > the issue, however imho
> > > > > > > the major issue is not about this single assertion
> > > > > > > but the whole addr_mask
> > > > > > > issue behind with virtio...
> > > > > > I don't get here, it looks to the the range was from
> > > > > > guest IOMMU drivers.
> > > > > Yes.  Note that I didn't mean that it's a problem in virtio,
> > > > > it's just the fact
> > > > > that virtio is the only one I know that would like to
> > > > > support arbitrary address
> > > > > range for the translated region.  I don't know about tcg,
> > > > > but vfio should still
> > > > > need some kind of page alignment in both the address and the
> > > > > addr_mask.  We
> > > > > have that assumption too across the memory core when we do
> > > > > translations.
> > > > 
> > > > Right but it looks to me the issue is not the alignment.
> > > > 
> > > > 
> > > > > A further cause of the issue is the MSI region when vIOMMU
> > > > > enabled - currently
> > > > > we implemented the interrupt region using another memory
> > > > > region so it split the
> > > > > whole DMA region into two parts.  That's really a clean approach to IR
> > > > > implementation, however that's also a burden to the
> > > > > invalidation part because
> > > > > then we'll need to handle things like this when the listened
> > > > > range is not page
> > > > > alighed at all (neither 0-0xfedffff, nor 0xfef0000-MAX).  If
> > > > > without the IR
> > > > > region (so the whole iommu address range will be a single FlatRange),
> > > > 
> > > > Is this a bug? I remember that at least for vtd, it won't do any
> > > > DMAR on the
> > > > intrrupt address range
> > > I don't think it's a bug, at least it's working as how I
> > > understand...  that
> > > interrupt range is using an IR region, that's why I said the IR
> > > region splits
> > > the DMAR region into two pieces, so we have two FlatRange for the same
> > > IOMMUMemoryRegion.
> > 
> > 
> > I don't check the qemu code but if "a single FlatRange" means
> > 0xFEEx_xxxx is subject to DMA remapping, OS need to setup passthrough
> > mapping for that range in order to get MSI to work. This is not what vtd
> > spec said:
> > 
> > """
> > 
> > 3.14 Handling Requests to Interrupt Address Range
> > 
> > Requests without PASID to address range 0xFEEx_xxxx are treated as
> > potential interrupt requests and are not subjected to DMA remapping
> > (even if translation structures specify a mapping for this
> > range). Instead, remapping hardware can be enabled to subject such
> > interrupt requests to interrupt remapping.
> > 
> > """
> > 
> > My understanding is vtd won't do any DMA translation on 0xFEEx_xxxx even
> > if IR is not enabled.
> 
> 
> Ok, we had a dedicated mr for interrupt:
> 
> memory_region_add_subregion_overlap(MEMORY_REGION(&vtd_dev_as->iommu),
> VTD_INTERRUPT_ADDR_FIRST,
> &vtd_dev_as->iommu_ir, 1);
> 
> So it should be fine. I guess the reason that I'm asking is that I thought
> "IR" means "Interrupt remapping" but in fact it means "Interrupt Region"?
> 
> But I'm still not clear about the invalidation part for interrupt region,
> maybe you can elaborate a little more on this.
> 
> Btw, I think guest can trigger the assert in vtd_do_iommu_translate() if we
> teach vhost to DMA to that region:


Why would we want to?

> 
>     /*
>      * We have standalone memory region for interrupt addresses, we
>      * should never receive translation requests in this region.
>      */
>     assert(!vtd_is_interrupt_addr(addr));
> 
> Is this better to return false here? (We can work on the fix for vhost but
> it should be not trivial)
> 
> Thanks
>