From 1decff3ff72e849a62c69fddacadca6b12307b7a Mon Sep 17 00:00:00 2001 From: Andrea Arcangeli <aarcange@redhat.com> Date: Tue, 20 Jan 2009 22:26:04 +0100 Subject: [PATCH 06/54] avoid cancelling ide dma The reason for not actually canceling the I/O is because with virtualization and lots of VM running, a guest fs may mistake a overload of the host, as an IDE timeout. So rather than canceling the I/O, it's safer to wait I/O completion and simulate that the I/O has completed just before the io cancellation was requested by the guest. This way if ntfs or an app writes data without checking for -EIO retval, and it thinks the write has succeeded, it's less likely to run into troubles. Similar issues for reads. Furthermore because the DMA operation is splitted into many synchronous aio_read/write if there's more than one entry in the SG table, without this patch the DMA would be cancelled in the middle, something we've no idea if it happens on real hardware too or not. Overall this seems a great risk for zero gain. This approach is sure safer than previous code given we can't pretend all guest fs code out there to check for errors and reply the DMA if it was completed partially, given a timeout would never materialize on a real harddisk unless there are defective blocks (and defective blocks are practically only an issue for reads never for writes in any recent hardware as writing to blocks is the way to fix them) or the harddisk breaks as a whole. Signed-off-by: Izik Eidus <ieidus@redhat.com> Signed-off-by: Andrea Arcangeli <aarcange@redhat.com> RH-Upstream-status: pending --- qemu/hw/ide.c | 21 +++++++++++++++++++-- 1 files changed, 19 insertions(+), 2 deletions(-) diff --git a/qemu/hw/ide.c b/qemu/hw/ide.c index 92ef4a5..9b725e6 100644 --- a/qemu/hw/ide.c +++ b/qemu/hw/ide.c @@ -2879,8 +2879,25 @@ static void bmdma_cmd_writeb(void *opaque, uint32_t addr, uint32_t val) printf("%s: 0x%08x\n", __func__, val); #endif if (!(val & BM_CMD_START)) { - /* XXX: do it better */ - ide_dma_cancel(bm); + /* + * We can't cancel Scatter Gather DMA in the middle of the + * operation or a partial (not full) DMA transfer would reach + * the storage so we wait for completion instead (we beahve + * like if the DMA was completed by the time the guest trying + * to cancel dma with bmdma_cmd_writeb with BM_CMD_START not + * set). + * + * In the future we'll be able to safely cancel the I/O if the + * whole DMA operation will be submitted to disk with a single + * aio operation with preadv/pwritev. + */ + if (bm->aiocb) { + qemu_aio_flush(); + if (bm->aiocb) + printf("ide_dma_cancel: aiocb still pending"); + if (bm->status & BM_STATUS_DMAING) + printf("ide_dma_cancel: BM_STATUS_DMAING still pending"); + } bm->cmd = val & 0x09; } else { if (!(bm->status & BM_STATUS_DMAING)) { -- 1.6.1