icount: make dma reads deterministic

icount: make dma reads deterministic

Windows guest sometimes makes DMA requests with overlapping
target addresses. This leads to the following structure of iov for
the block driver:

addr size1
addr size2
addr size3

It means that three adjacent disk blocks should be read into the same
memory buffer. Windows does not expects anything from these bytes
(should it be data from the first block, or the last one, or some mix),
but uses them somehow. It leads to non-determinism of the guest execution,
because block driver does not preserve any order of reading.

This situation was discusses in the mailing list at least twice:
https://lists.gnu.org/archive/html/qemu-devel/2010-09/msg01996.html
https://lists.gnu.org/archive/html/qemu-devel/2020-02/msg05185.html

This patch makes such disk reads deterministic in icount mode.
It splits the whole request into several parts. Parts may overlap,
but SGs inside one part do not overlap.
Parts that are processed later overwrite the prior ones in case
of overlapping.

Examples for different SG part sequences:

1)
A1 1000
A2 1000
A1 1000
A3 1000
->
One request is split into two.
A1 1000
A2 1000
--
A1 1000
A3 1000

2)
A1 800
A2 1000
A1 1000
->
A1 800
A2 1000
--
A1 1000

Signed-off-by: Pavel Dovgalyuk <Pavel.Dovgaluk@ispras.ru>
Message-Id: <159117972206.12193.12939621311413561779.stgit@pasha-ThinkPad-X280>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>