Scott Wood wrote: > On Tue, 14 Dec 2010 01:24:50 +0100 > Alexander Graf <alex@xxxxxxxxx> wrote: > > >> On 14.12.2010, at 01:18, Scott Wood wrote: >> >> >>> Right, but I'm not talking about an interrupt that happens when the >>> virtual EE bit is zero. I'm talking about an interrupt that happens >>> right in the middle of the paravirt sequence -- after reading int_pending, >>> but before setting critical to r2. >>> >>> It seems like the race window is just narrowed, not eliminated. >>> >> Hrm, is that window really that important? There's usually plenty of interrupts and mmios coming through to always have some check going on. >> > > It could be important for realtime loads, tickless systems (especially > if the Linux host eventually grows the ability to be tickless even > when things are running), etc., and it makes me nervous in general. > > It's not something that's going to be causing problems all the time, > though. > I agree - it's certainly wrong. >> If it really is important, we could also check int_pending right after the critical section and just do a nop exit. >> > > Doesn't checking int_pending require clobbering registers, which is why > we have the critical section in the first place? > The critical section is to prevent us from overwriting the scratch registers, yeah. And I think you're right - I had a thinko last night. If we see that we should inject an interrupt, but we're inside of a critical section, we could set the magic page to r/o and try to find the critical end at which point we can just inject. > >> That way we worst case waste a few cycles for the useless guest exit, >> but always fetch interrupts immediately when they occur. >> > > What useless guest exit? Either we exit when we see an interrupt > pending (in which case it's not useless), or we exit all the time, and > then what's the point of the paravirt? > I was thinking of a case where we get a few false positives. But again, I probably just had a bad thought :) Alex -- To unsubscribe from this list: send the line "unsubscribe kvm-ppc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
