Re: [Qemu-arm] [PATCH for-4.2 v10 08/15] virtio-iommu: Implement map/unmap

[Tian, Kevin]

> From: Peter Xu [mailto:address@hidden]
> Sent: Wednesday, September 4, 2019 1:37 PM
> 
> On Wed, Sep 04, 2019 at 04:23:50AM +0000, Tian, Kevin wrote:
> > > From: Peter Xu [mailto:address@hidden]
> > > Sent: Wednesday, September 4, 2019 9:44 AM
> > >
> > > On Tue, Sep 03, 2019 at 01:37:11PM +0200, Auger Eric wrote:
> > > > Hi Peter,
> > > >
> > > > On 8/19/19 10:11 AM, Peter Xu wrote:
> > > > > On Tue, Jul 30, 2019 at 07:21:30PM +0200, Eric Auger wrote:
> > > > >
> > > > > [...]
> > > > >
> > > > >> +    mapping = g_tree_lookup(domain->mappings,
> (gpointer)(&interval));
> > > > >> +
> > > > >> +    while (mapping) {
> > > > >> +        viommu_interval current;
> > > > >> +        uint64_t low  = mapping->virt_addr;
> > > > >> +        uint64_t high = mapping->virt_addr + mapping->size - 1;
> > > > >> +
> > > > >> +        current.low = low;
> > > > >> +        current.high = high;
> > > > >> +
> > > > >> +        if (low == interval.low && size >= mapping->size) {
> > > > >> +            g_tree_remove(domain->mappings, (gpointer)(&current));
> > > > >> +            interval.low = high + 1;
> > > > >> +            trace_virtio_iommu_unmap_left_interval(current.low,
> > > current.high,
> > > > >> +                interval.low, interval.high);
> > > > >> +        } else if (high == interval.high && size >= mapping->size) {
> > > > >> +            trace_virtio_iommu_unmap_right_interval(current.low,
> > > current.high,
> > > > >> +                interval.low, interval.high);
> > > > >> +            g_tree_remove(domain->mappings, (gpointer)(&current));
> > > > >> +            interval.high = low - 1;
> > > > >> +        } else if (low > interval.low && high < interval.high) {
> > > > >> +            trace_virtio_iommu_unmap_inc_interval(current.low,
> > > current.high);
> > > > >> +            g_tree_remove(domain->mappings, (gpointer)(&current));
> > > > >> +        } else {
> > > > >> +            break;
> > > > >> +        }
> > > > >> +        if (interval.low >= interval.high) {
> > > > >> +            return VIRTIO_IOMMU_S_OK;
> > > > >> +        } else {
> > > > >> +            mapping = g_tree_lookup(domain->mappings,
> > > (gpointer)(&interval));
> > > > >> +        }
> > > > >> +    }
> > > > >> +
> > > > >> +    if (mapping) {
> > > > >> +        qemu_log_mask(LOG_GUEST_ERROR,
> > > > >> +                      "****** %s: Unmap 0x%"PRIx64" size=0x%"PRIx64
> > > > >> +                     " from 0x%"PRIx64" size=0x%"PRIx64" is not 
> > > > >> supported\n",
> > > > >> +                     __func__, interval.low, size,
> > > > >> +                     mapping->virt_addr, mapping->size);
> > > > >> +    } else {
> > > > >> +        return VIRTIO_IOMMU_S_OK;
> > > > >> +    }
> > > > >> +
> > > > >> +    return VIRTIO_IOMMU_S_INVAL;
> > > > >
> > > > > Could the above chunk be simplified as something like below?
> > > > >
> > > > >   while ((mapping = g_tree_lookup(domain->mappings, &interval))) {
> > > > >     g_tree_remove(domain->mappings, mapping);
> > > > >   }
> > > > Indeed the code could be simplified. I only need to make sure I don't
> > > > split an existing mapping.
> > >
> > > Hmm... Do we need to still split an existing mapping if necessary?
> > > For example when with this mapping:
> > >
> > >   iova=0x1000, size=0x2000, phys=ADDR1, flags=FLAGS1
> > >
> > > And if we want to unmap the range (iova=0, size=0x2000), then we
> > > should split the existing mappping and leave this one:
> > >
> > >   iova=0x2000, size=0x1000, phys=(ADDR1+0x1000), flags=FLAGS1
> > >
> > > Right?
> > >
> >
> > virtio-iommu spec explicitly disallows partial unmap.
> >
> > 5.11.6.6.1 Driver Requirements: UNMAP request
> >
> > The first address of a range MUST either be the first address of a
> > mapping or be outside any mapping. The last address of a range
> > MUST either be the last address of a mapping or be outside any
> > mapping.
> >
> > 5.11.6.6.2 Device Requirements: UNMAP request
> >
> > If a mapping affected by the range is not covered in its entirety
> > by the range (the UNMAP request would split the mapping),
> > then the device SHOULD set the request status to VIRTIO_IOMMU
> > _S_RANGE, and SHOULD NOT remove any mapping.
> 
> I see, thanks Kevin.
> 
> Though why so strict?  (Sorry if I missed some discussions
> ... pointers welcomed...)
> 
> What I'm thinking is when we want to allocate a bunch of buffers
> (e.g., 1M) while we will also need to be able to free them with
> smaller chunks (e.g., 4K), then it would be even better that we allow
> to allocate a whole 1M buffer within the guest and map it as a whole,
> then we can selectively unmap the pages after used.  If with the
> strict rule, we'll need to map one by one, that can be a total of
> 1M/4K roundtrips.
> 

Sorry I forgot the original discussion. Need Jean to respond. :-)

A possible reason is that no such usage exists today, thus simplification
was made? 

Thanks
Kevin