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

[Peter Xu <peterx@redhat.com>]

On Wed, Sep 04, 2019 at 04:23:50AM +0000, Tian, Kevin wrote:
> > From: Peter Xu [mailto:peterx@redhat.com]
> > 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.

Regards,

-- 
Peter Xu