Re: [PATCH v4] icount: make dma reads deterministic

[Kevin Wolf <kwolf@redhat.com>]

Am 03.06.2020 um 12:22 hat Pavel Dovgalyuk geschrieben:
> From: Pavel Dovgalyuk <pavel.dovgaluk@gmail.com>
> 
> 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>

Thanks, applied to the block branch.

Kevin