[Xen-changelog] [qemu-upstream-unstable] serial: reset thri_pending on IER writes with THRI=0

[patchbot@xxxxxxx]

commit e54bcad901f4891dd5c3bf72e199175a23944214
Author:     Paolo Bonzini <pbonzini@xxxxxxxxxx>
AuthorDate: Fri Dec 12 10:17:08 2014 +0100
Commit:     Michael Roth <mdroth@xxxxxxxxxxxxxxxxxx>
CommitDate: Sun Feb 22 18:03:34 2015 -0600

    serial: reset thri_pending on IER writes with THRI=0
    
    This is responsible for failure of migration from 2.2 to 2.1, because
    thr_ipending is always one in practice.
    
    serial.c is setting thr_ipending unconditionally.  However, thr_ipending
    is not used at all if THRI=0, and it will be overwritten again the next
    time THRE or THRI changes.  For that reason, we can set thr_ipending to
    zero every time THRI is reset.
    
    There is disagreement on whether LSR.THRE should be resampled when IER.THRI
    goes from 1 to 1.  This patch does not touch the code, leaving that for
    QEMU 2.3+.
    
    This has no semantic change and is enough to fix migration in the common
    case where the interrupt is not pending or is reported in IIR.  It does not
    change the migration format, so 2.2.0 -> 2.1 will remain broken but we
    can fix 2.2.1 -> 2.1 without breaking 2.2.1 <-> 2.2.0.
    
    The case that remains broken (the one in which the subsection is strictly
    necessary) is when THRE=1, the THRI interrupt has *not* been acknowledged
    yet, and a higher-priority interrupt comes.  In this case, you need the
    subsection to tell the source that the lower-priority THRI interrupt is
    pending.  The subsection's breakage of migration, in this case, prevents
    continuing the VM on the destination with an invalid state.
    
    Cc: qemu-stable@xxxxxxxxxx
    Reported-by: Igor Mammedov <imammedo@xxxxxxxxxx>
    Reviewed-by: Dr. David Alan Gilbert <dgilbert@xxxxxxxxxx>
    Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
    (cherry picked from commit 4e02b0fcf5c97579d0d3261c80c65abcf92870fe)
    Signed-off-by: Michael Roth <mdroth@xxxxxxxxxxxxxxxxxx>
---
 hw/char/serial.c |   18 ++++++++++++++++--
 1 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/hw/char/serial.c b/hw/char/serial.c
index ebcacdc..8c42d03 100644
--- a/hw/char/serial.c
+++ b/hw/char/serial.c
@@ -350,10 +350,24 @@ static void serial_ioport_write(void *opaque, hwaddr 
addr, uint64_t val,
                      s->poll_msl = 0;
                 }
             }
-            if (s->lsr & UART_LSR_THRE) {
+
+            /* Turning on the THRE interrupt on IER can trigger the interrupt
+             * if LSR.THRE=1, even if it had been masked before by reading IIR.
+             * This is not in the datasheet, but Windows relies on it.  It is
+             * unclear if THRE has to be resampled every time THRI becomes
+             * 1, or only on the rising edge.  Bochs does the latter, and 
Windows
+             * always toggles IER to all zeroes and back to all ones.  But for
+             * now leave it as it has always been in QEMU.
+             *
+             * If IER.THRI is zero, thr_ipending is not used.  Set it to zero
+             * so that the thr_ipending subsection is not migrated.
+             */
+            if ((s->ier & UART_IER_THRI) && (s->lsr & UART_LSR_THRE)) {
                 s->thr_ipending = 1;
-                serial_update_irq(s);
+            } else {
+                s->thr_ipending = 0;
             }
+            serial_update_irq(s);
         }
         break;
     case 2:
--
generated by git-patchbot for /home/xen/git/qemu-upstream-unstable.git

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