[Qemu-devel] [RFC] e1000: Don't save writes to ICS/ICR masked by IMS

[Ed Swierk]

Windows 8, 10 and Server 2012 guests hang intermittently while booting
on Xen 4.5.3 with 1 vCPU and 4 e1000 vNICs, shortly after the Windows
logo appears and the little dots start spinning.

Running strace on qemu shows its main thread doing the following every
couple of milliseconds:

 ppoll([..., {fd=30</dev/xen/evtchn>, events=POLLIN|POLLERR|POLLHUP},
        ...], ...) = 1 ([{fd=30, revents=POLLIN}], ...)
 read(30</dev/xen/evtchn>, "^\0\0\0", 4) = 4
 write(30</dev/xen/evtchn>, "^\0\0\0", 4) = 4
 ioctl(30</dev/xen/evtchn>, IOCTL_EVTCHN_NOTIFY, 0x7f1f9449d310) = 0
 clock_gettime(CLOCK_MONOTONIC, {6937, 449468262}) = 0
 clock_gettime(CLOCK_MONOTONIC, {6937, 449582903}) = 0
 gettimeofday({1472251376, 673434}, NULL) = 0
 clock_gettime(CLOCK_MONOTONIC, {6937, 449856205}) = 0
 gettimeofday({1472251376, 673679}, NULL) = 0

The event channel (identified by '^' or 94 in this example) is always
the third of the domain's four channels.

Two recent qemu patches (http://git.qemu.org/?p=qemu.git;h=9596ef7c and
http://git.qemu.org/?p=qemu.git;h=74004e8c) seem to address similar
issues, but don't help in this case.

The proposed fix from
https://bugzilla.redhat.com/show_bug.cgi?id=874406#c78 makes the hang
go away. It's not clear to me why it works, or if it's just papering
over a bug elsewhere, or if there are any possible side effects.

Suggested-by: Andrew Jones <address@hidden>
Signed-off-by: Ed Swierk <address@hidden>

diff --git a/hw/net/e1000.c b/hw/net/e1000.c
index 6eac66d..c891b67 100644
--- a/hw/net/e1000.c
+++ b/hw/net/e1000.c
@@ -293,6 +293,8 @@ set_interrupt_cause(E1000State *s, int index, uint32_t val)
     uint32_t pending_ints;
     uint32_t mit_delay;
 
+    val &= s->mac_reg[IMS];
+
     s->mac_reg[ICR] = val;
 
     /*
@@ -305,7 +307,7 @@ set_interrupt_cause(E1000State *s, int index, uint32_t val)
      */
     s->mac_reg[ICS] = val;
 
-    pending_ints = (s->mac_reg[IMS] & s->mac_reg[ICR]);
+    pending_ints = s->mac_reg[ICR];
     if (!s->mit_irq_level && pending_ints) {
         /*
          * Here we detect a potential raising edge. We postpone raising the