On 22.05.2012, at 07:03, Rusty Russell <rusty.russell at linaro.org> wrote: > On Tue, 22 May 2012 05:27:28 +0200, Alexander Graf <agraf at suse.de> wrote: >> On 22.05.2012, at 05:00, Rusty Russell <rusty.russell at linaro.org> wrote: >>> Note that in some cases, we could skip saving and restoring the host FPU >>> state. But that seems like a premature optimization, >> >> Yup, getting things work first and fast later is usually the better >> approach :). > > Actually, I prefer getting it to work and putting a FIXME. Then someone > else can do all the tricky work ;) > >> How does the ARM code work here? Are you guys doing it x86 style and exit vcpu_run for every intercept or are you going the ppc way to keep inside vcpu_run until we want to return to user space? >> >> The former benefits a lot from not saving/restoring the host fpu state. The latter not so much :). > > The latter. So this is how it looks at the moment, assuming the guest > always hits the FPU, we do a hyp trap and two fpu switches for each > guest entry. > > Now, we could do some heuristics about FPU-intense guests, and try to > avoid the hyp trap by always switching the FPU. We won't know the costs > involved until we benchmark on real silicon. Can't you just keep the FPU in guest context until you hit vcpu_put? Check out the ppc code for 'ext' handling - it's fairly simple. > >>> and makes us >>> depend on the specific Linux kernel saving behavior. >> >> ... which should be a reasonable thing to do given that kvm is part of >> the Linux kernel ;). > > Except the it allows changes in arch/arm/vfp/ to break code in > arch/arm/kvm/. Yeah, but that will always be the case, no? Changes in oprofile could also hit you when you suddenly get intercepts you don't know yet. > > And we won't spot it for ages, because it's subtle and only breaks in > odd corner cases. This seems to be the history of floating point. Well - corner cases are bad of course :) > > Worse, we can't call kernel code to frob the FPU, since we want to stay > in hyp mode to do the switch. :( What's the reason we can't run all of Linux in HV mode like we do on ppc again? ;) Alex
