Re: [Qemu-devel] [PATCH] e1000: Delay LSC until mask is active

[Alexander Graf <agraf@suse.de>]

On 03.07.14 19:57, Peter Maydell wrote:
> On 3 July 2014 18:39, Alexander Graf <agraf@suse.de> wrote:
>> Mac OS X reads ICR on every interrupt. When the IRQ line is shared, this may
>> result in a race where LSC is not interpreted yet, but already gets cleared.
>>
>> The guest already has a way of telling us that it can interpret LSC events
>> though and that's via the interrupt mask register (IMS).
>>
>> So if we just leave the LSC interrupt bit pending, but invisible to the guest
>> as long as it's not ready to receive LSC interrupts, we basically defer the
>> interrupt to the earliest point in time when the guest would know how to
>> handle it.
> This would break any guests dealing with this in a polling
> mode (ie "permanently leave interrupts masked and read
> ICR periodically to find out whether anything interesting
> has happened"), right?

If those guests would wait for a link detect event that way, yes.

Considering all the hackery we already have about link negotiation 
(delay it until a random amount of ms passed) I'd say the breakage this 
patch fixes is a lot more likely than a polling guest that waits for a 
link based on ICR.LSC :).


Alex