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

[Jason Wang]

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:

    /*
     * 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