On Wed, Sep 12, 2018 at 01:21:57PM +0200, Christoffer Dall wrote: > On Wed, Sep 12, 2018 at 12:16:26PM +0100, Dave Martin wrote: > > On Wed, Sep 12, 2018 at 12:17:04PM +0200, Christoffer Dall wrote: [...] > > > I think the point was actually to avoid saving/restoring FPEXC32_EL2 > > > when VFP was being trapped to EL2 > > > > With what purpose in mind? > > I don't think we thought it through very carefully but just considered > FPEXC32_EL2 as part of the VFP state, but as you say, that doesn't > really make sense. > > I was just objecting to the statement that this is a bring-over from the > 32-bit side, which confused me, and I don't think that's true. [...] > > We saved neither the trap (since the VFP registers needed switching > > anyway) nor the FPEXC32_EL2 switch (since we had to do that at guest > > entry). > > > > I may have misunderstood something. > > > > No, we just didn't look at it that carefully back then. > > > > > I was somewhat guessing likely rationale, so if you can suggest a > > way to reword this that reflects the situation better I'd be happy > > to change it. > > "Christoffer was being stupid..." ;) It was just the way > it was, I don't think there was a good rationale, but I don't think it > helps to understand the change to provide guesswork either, so just > remove it. OK, I'll drop the bogus history lesson from this patch. It doesn't really add anything here, especially if it's wrong. > > Thinking about it, I don't see why FPEXC32_EL2 can't be handled via > > vcpu_load()/_put(). This register doesn't matter for the host, and > > I can't see why we would need to switch it when going through hyp. > > > > That's another good point, should be able to do that. [...] > > I confess to a certain amount of "according to the spirit of the > > architecture it obviously ought to work this way" reasoning. > > > > Following the trail of documentation isn't trivial, so it would > > be good if someone could check that they can reach the same conclusion > > about FPEXC32_EL2 having no effect on the host. > > > > That was definitely my conclusion as well. OK, sounds good. [...] > > > We can queue this for the next merge window. > > > > > > Reviewed-by: Christoffer Dall <christoffer.dall@xxxxxxx> > > > > Thanks, modulo the question of maybe moving this to the vcpu_load()/ > > _put() path. > > > > Sure, I'll look at the next version as well if you send a new version. Will do. Can't say exactly when. Cheers ---Dave _______________________________________________ kvmarm mailing list kvmarm@xxxxxxxxxxxxxxxxxxxxx https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
