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Re: [Qemu-block] [Qemu-devel] [PATCH V3 4/6] ide: orphan all buffered re

From: Peter Lieven
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH V3 4/6] ide: orphan all buffered requests on DMA cancel
Date: Thu, 12 Nov 2015 09:45:32 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.3.0

Am 12.11.2015 um 09:27 schrieb Fam Zheng:
On Fri, 11/06 09:42, Peter Lieven wrote:
If the guests canceles a DMA request we can prematurely
invoke all callbacks of buffered requests and flag all them
as orphaned. Ideally this avoids the need for draining all
requests. For CDROM devices this works in 100% of all cases.

Signed-off-by: Peter Lieven <address@hidden>
  hw/ide/pci.c | 19 +++++++++++++++++++
  1 file changed, 19 insertions(+)

diff --git a/hw/ide/pci.c b/hw/ide/pci.c
index d31ff88..a9e164e 100644
--- a/hw/ide/pci.c
+++ b/hw/ide/pci.c
@@ -240,6 +240,22 @@ void bmdma_cmd_writeb(BMDMAState *bm, uint32_t val)
      /* Ignore writes to SSBM if it keeps the old value */
      if ((val & BM_CMD_START) != (bm->cmd & BM_CMD_START)) {
          if (!(val & BM_CMD_START)) {
+            /* First invoke the callbacks of all buffered requests
+             * and flag those requests as orphaned. Ideally there
+             * are no unbuffered (Scatter Gather DMA Requests or
+             * write requests) pending and we can avoid to drain. */
+            IDEBufferedRequest *req;
+            IDEState *s = idebus_active_if(bm->bus);
+            QLIST_FOREACH(req, &s->buffered_requests, list) {
+                if (!req->orphaned) {
+#ifdef DEBUG_IDE
+                    printf("%s: invoking cb %p of buffered request %p with"
+                           " -ECANCELED\n", __func__, req->original_cb, req);
+                    req->original_cb(req->original_opaque, -ECANCELED);
+                }
+                req->orphaned = true;
+            }
Why not use bdrv_aio_cancel or bdrv_aio_cancel_async with the aio returned by

bdrv_aio_cancel would block until the request is completed, that wouldn't help 
the storage is no longer responsive.

The trick with the buffered request is that we can avoid waiting for the 
storage and
guarantee that a later completion on the storage won't corrupt guest memory.


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