Re: [Qemu-devel] [PATCH] memory: do not do out of bound notification

[Peter Xu]

On Wed, Jun 19, 2019 at 03:17:41PM +0200, Auger Eric wrote:
> Hi Yan,
> 
> [+ Peter]
> 
> On 6/19/19 10:49 AM, Yan Zhao wrote:
> > even if an entry overlaps with notifier's range, should not map/unmap
> > out of bound part in the entry.
> 
> I don't think the patch was based on the master as the trace at the very
> end if not part of the upstream code.
> > 
> > This would cause problem in below case:
> > 1. initially there are two notifiers with ranges
> > 0-0xfedfffff, 0xfef00000-0xffffffffffffffff,
> > IOVAs from 0x3c000000 - 0x3c1fffff is in shadow page table.
> > 
> > 2. in vfio, memory_region_register_iommu_notifier() is followed by
> > memory_region_iommu_replay(), which will first call address space unmap,
> > and walk and add back all entries in vtd shadow page table. e.g.
> > (1) for notifier 0-0xfedfffff,
> >     IOVAs from 0 - 0xffffffff get unmapped,
> >     and IOVAs from 0x3c000000 - 0x3c1fffff get mapped
> 
> While the patch looks sensible, the issue is the notifier scope used in
> vtd_address_space_unmap is not a valid mask (ctpop64(size) != 1). Then
> the size is recomputed (either using n = 64 - clz64(size) for the 1st
> notifier or n = s->aw_bits for the 2d) and also the entry (especially
> for the 2d notifier where it becomes 0) to get a proper alignment.
> 
> vtd_page_walk sends notifications per block or page (with valid
> addr_mask) so stays within the notifier.
> 
> Modifying the entry->iova/addr_mask again in memory_region_notify_one
> leads to unaligned start address / addr_mask. I don't think we want that.
> 
> Can't we modity the vtd_address_space_unmap() implementation to split
> the invalidation in smaller chunks instead?

Seems workable, to be explicit - we can even cut it into chunks with
different size to be efficient.  Like, this range:

  0x0e00_0000 - 0x1_0000_0000 (size 0xf200_0000)

can be one of this:

  0x0e000000 - 0x1000_0000 (size 0x0200_0000)

plus one of this:

  0x1000_0000 - 0x1_0000_0000 (size 0xf000_0000)

Yan, could you help explain the issue better on how to reproduce and
what's the error when the problem occurs?  For example, is that
happened when a device hot-plugged into an existing VFIO container
(with some mapped IOVAs)?  Did you get host DMA errors later on?

Thanks,

-- 
Peter Xu