On Mon, Dec 11, 2017 at 08:24:32PM +0100, Christoffer Dall wrote: > On Mon, Dec 11, 2017 at 02:51:36PM +0000, Dave Martin wrote: > > On Fri, Nov 24, 2017 at 03:45:38PM +0100, Christoffer Dall wrote: [...] > > > So you're saying even if we try the "expose full width and read back > > > hidden values" approach, those hidden values may be changed when > > > executing the guest, due to the KVM implementation or the way hardware > > > works, is that the point? > > > > Basically yes. > > > > > I think the KVM interface should be designed similarly to being able to > > > probe a hardware CPU's register state at various stages of execution. > > > > > > So, for example, if you write content to hidden bits in the SVE > > > registers from EL2 on real hardware and limit the length using ZCR_EL2, > > > and then run a bunch of code in EL1/0, and then come back to EL2 and > > > examine the registers again, then we should model that behavior in > > > software. > > > > > > In other words, I think we have to model this more closely to what > > > guarantees ZCR_EL2 gives us, and not ZCR_EL1, and choose something > > > architecturally compliant which is reasonable to implement. > > > > So, we imagine that provided the vcpu is not run in the meantime, > > all accesses to SVE regs via the KVM reg API act like they are executed > > at EL2? > > Yes, userspace probing virtual EL1 state should be like EL2 probing EL1 > state on hardware. > > > > > That doesn't seem unreasonable, and it removes any ordering requirement > > between ZCR_EL1 and the SVE regs providing that the vcpu isn't set > > running in the meantime. There is no userspace access to ZCR_EL2 at > > all, if we go with the model of configuring that via attributes that > > must be configured before vcpu startup -- in which case there is no > > ordering requirement there. > > > > The extra bits beyond ZCR_EL1.LEN may disappear as soon as the vcpu > > is run, but that is architecturally consistent behaviour at least. > > > > Yes, I think we agree here. It will all be interesting with nested > virtualization where we have to start exposing ZCR_EL2, but that's not > for today. OK, that sounds reasonable. There are a couple of options open for the nested virt case, but we don't need to worry about it now in any case. Cheers ---Dave _______________________________________________ kvmarm mailing list kvmarm@xxxxxxxxxxxxxxxxxxxxx https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
