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

[Paolo Bonzini]

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.

Paolo