Re: [Qemu-devel] [PATCH] implement moderation registers for e1000

[Luigi Rizzo]

On Fri, Feb 08, 2013 at 12:52:12PM +0100, Paolo Bonzini wrote:
> Il 07/02/2106 07:28, Luigi Rizzo ha scritto:
> > On Fri, Feb 08, 2013 at 11:46:34AM +0100, Paolo Bonzini wrote:
> >> Il 08/02/2013 11:20, Luigi Rizzo ha scritto:
> > ...
> >>> I am happy to make this default to off. But it would be good if you could
> >>> actually give it a try. Note that linux and FreeBSD (and presumably 
> >>> windows)
> >>> do use moderation by default so enabling the emulation of the
> >>> registers makes the guest OS behave differently (and closer to bare 
> >>> metal).
> >>>
> >>> To test that the patch itself does not cause regression in the emulator
> >>> one should also turn off moderation in the guest (one of the tests i
> >>> have run).
> >>
> >> I think making the default on is fine, but you need to add compatibility
> >> options to leave it off in older machine types (pc-1.4 and earlier).
> > 
> > I am unclear on what you mean by "older machine types" ?
> > The hardware (E1000_DEV_ID_82540EM) does have these registers,
> > and it is entirely up to the guest OS to use them or not.
> 
> Yes, but suppose a guest has a bug that was masked by the old
> non-implementation.  "-M pc-1.4" is supposed to bring back the old
> version's behavior as much as possible, including making the guest bug
> latent again.

ok i see. Is there some example code that already handles
a similar '-M' option so i can look at it and use to set
the mit_on parameter ?

cheers
luigi