[Qemu-devel] [PATCH 36/99] ahci: fix PxCI register race

[Michael Roth <mdroth@linux.vnet.ibm.com>]

From: John Snow <jsnow@redhat.com>

Fixes: https://bugs.launchpad.net/qemu/+bug/1769189

AHCI presently signals completion prior to the PxCI register being
cleared to indicate completion. If a guest driver attempts to issue
a new command in its IRQ handler, it might be surprised to learn there
is still a command pending.

In the case of Windows 10's boot driver, it will actually poll the IRQ
register hoping to find out when the command is done running -- which
will never happen, as there isn't a command running.

Fix this: clear PxCI in ahci_cmd_done and not in the asynchronous BH.
Because it now runs synchronously, we don't need to check if the command
is actually done by spying on the ATA registers. We know it's done.

CC: qemu-stable <qemu-stable@nongnu.org>
Reported-by: François Guerraz <kubrick@fgv6.net>
Tested-by: Bruce Rogers <brogers@suse.com>
Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Message-id: 20180531004323.4611-3-jsnow@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>
(cherry picked from commit 5694c7eacce6b263ad7497cc1bb76aad746cfd4e)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
---
 hw/ide/ahci.c | 13 ++++++-------
 1 file changed, 6 insertions(+), 7 deletions(-)

diff --git a/hw/ide/ahci.c b/hw/ide/ahci.c
index e22d7be05f..18b9a9c18b 100644
--- a/hw/ide/ahci.c
+++ b/hw/ide/ahci.c
@@ -532,13 +532,6 @@ static void ahci_check_cmd_bh(void *opaque)
     qemu_bh_delete(ad->check_bh);
     ad->check_bh = NULL;
 
-    if ((ad->busy_slot != -1) &&
-        !(ad->port.ifs[0].status & (BUSY_STAT|DRQ_STAT))) {
-        /* no longer busy */
-        ad->port_regs.cmd_issue &= ~(1 << ad->busy_slot);
-        ad->busy_slot = -1;
-    }
-
     check_cmd(ad->hba, ad->port_no);
 }
 
@@ -1425,6 +1418,12 @@ static void ahci_cmd_done(IDEDMA *dma)
 
     trace_ahci_cmd_done(ad->hba, ad->port_no);
 
+    /* no longer busy */
+    if (ad->busy_slot != -1) {
+        ad->port_regs.cmd_issue &= ~(1 << ad->busy_slot);
+        ad->busy_slot = -1;
+    }
+
     /* update d2h status */
     ahci_write_fis_d2h(ad);
 
-- 
2.17.1