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Pavel Dovgalyuk > -----Original Message----- > From: Pavel Dovgalyuk [mailto:[email protected]] > Sent: Tuesday, March 03, 2020 3:27 PM > To: [email protected] > Cc: [email protected]; [email protected]; [email protected]; > [email protected]; > [email protected] > Subject: [PATCH v2] 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 <[email protected]> > > -- > > v2: > - Rewritten the loop to split the request instead of skipping the parts > (suggested by Kevin Wolf) > --- > dma-helpers.c | 19 +++++++++++++++++++ > 1 file changed, 19 insertions(+) > > diff --git a/dma-helpers.c b/dma-helpers.c > index e8a26e81e1..959e114595 100644 > --- a/dma-helpers.c > +++ b/dma-helpers.c > @@ -13,6 +13,8 @@ > #include "trace-root.h" > #include "qemu/thread.h" > #include "qemu/main-loop.h" > +#include "sysemu/cpus.h" > +#include "qemu/range.h" > > /* #define DEBUG_IOMMU */ > > @@ -142,6 +144,23 @@ static void dma_blk_cb(void *opaque, int ret) > cur_addr = dbs->sg->sg[dbs->sg_cur_index].base + dbs->sg_cur_byte; > cur_len = dbs->sg->sg[dbs->sg_cur_index].len - dbs->sg_cur_byte; > mem = dma_memory_map(dbs->sg->as, cur_addr, &cur_len, dbs->dir); > + /* > + * Make reads deterministic in icount mode. Windows sometimes issues > + * disk read requests with overlapping SGs. It leads > + * to non-determinism, because resulting buffer contents may be mixed > + * from several sectors. This code splits all SGs into several > + * groups. SGs in every group do not overlap. > + */ > + if (use_icount && dbs->dir == DMA_DIRECTION_FROM_DEVICE) { > + int i; > + for (i = 0 ; i < dbs->iov.niov ; ++i) { > + if (ranges_overlap((intptr_t)dbs->iov.iov[i].iov_base, dbs- > >iov.iov[i].iov_len, > + (intptr_t)mem, cur_len)) { > + mem = NULL; > + break; > + } > + } > + } > if (!mem) > break; > qemu_iovec_add(&dbs->iov, mem, cur_len);
