On Wed, Jan 09, 2013 at 11:36:50PM +0800, Jason Wang wrote: > On 01/09/2013 11:34 PM, Michael S. Tsirkin wrote: > > On Wed, Jan 09, 2013 at 11:28:29PM +0800, Jason Wang wrote: > >> On 01/09/2013 06:51 PM, Michael S. Tsirkin wrote: > >>> Since commit b1332393cdd7d023de8f1f8aa136ee7866a18968, > >>> qemu started updating ICS register when interrupt > >>> is sent, with the intent to match spec better > >>> (guests do not actually read this register). > >>> However, the function set_interrupt_cause where ICS > >>> is updated is often called internally by > >>> device emulation so reading it does not produce the last value > >>> written by driver. Looking closer at the spec, > >>> it documents ICS as write-only, so there's no need > >>> to update it at all. I conclude that while harmless this line is useless > >>> code so removing it is a bit cleaner than keeping it in. > >>> > >>> Tested with windows and linux guests. > >>> > >>> Cc: Bill Paul <wp...@windriver.com> > >>> Reported-by: Yan Vugenfirer <y...@daynix.com> > >>> Signed-off-by: Michael S. Tsirkin <m...@redhat.com> > >>> --- > >>> hw/e1000.c | 1 - > >>> 1 file changed, 1 deletion(-) > >>> > >>> diff --git a/hw/e1000.c b/hw/e1000.c > >>> index 92fb00a..928d804 100644 > >>> --- a/hw/e1000.c > >>> +++ b/hw/e1000.c > >>> @@ -230,7 +230,6 @@ set_interrupt_cause(E1000State *s, int index, > >>> uint32_t val) > >>> val |= E1000_ICR_INT_ASSERTED; > >>> } > >>> s->mac_reg[ICR] = val; > >>> - s->mac_reg[ICS] = val; > >>> qemu_set_irq(s->dev.irq[0], (s->mac_reg[IMS] & s->mac_reg[ICR]) != > >>> 0); > >>> } > >>> > >> If my memory is correct, though ICS is marked as read only in the spec, > >> we do can read it when I'm examining a real e1000 card. > > Interesting, this was not Bill's motivation. > > I haven't seen any reads with linux or windows guests - > > which guest did trigger them for you? > > Also, what's the value one would expect? > > > > I also find this violation of spec in the past, so I just hack the linux > driver to see the result. I'm not sure if there are some driver depends > on this value. I forget the detail of what value it returns, may worth > to try again to see.
What Bill wrote in the commit log is that he didn't see any guest reading this. -- MST
