On 20/01/2016 16:20, 'Roman Kagan' wrote: >> > Because, as the docs say, we don't want to do that. We want to use >> > KVM_EXIT_IO or KVM_EXIT_MMIO, with two exceptions: s390 and wherever we >> > can't do that for compatibility purposes. > I must admit I saw this part in the docs but failed to understand it. > > Is it supposed to mean that we want to avoid using hypercalls as a means > of guest communications with the host userspace in general, and use PIO > or MMIO instead, unless hypercalls are mandated by a guest implemenation > we can't affect (as is the case for Hyper-V)? In this case we use hypercalls. :) However, we use a separate exit instead of putting everything under KVM_EXIT_HYPERCALL. This, as you mentioned, has the advantage that you can customize it according to the calling convention (which may help avoiding TOCTTOU issues, too). > Because *implementing* Hyper-V hypercalls in terms of KVM_EXIT_IO or > KVM_EXIT_MMIO looked hard at best. > >> > So we should not add a new exit > Why? VCPU exit codes are not a scarse resource. Indeed, but grouping makes things easier to understand. > So far we've envisaged two reasons for VCPU exit related to hyper-v: one > for hyper-v MSRs and the other for hypercalls. Since there was a > discussion on implementing generic MSR access by Peter we thought it > wiser to introduce a new VCPU exit for hyper-v hypercalls to avoid > interfering with the MSR implementation. That's a good idea. However, I think I'm not going to accept the MSR exit feature, and then the current Hyper-V exit API makes some sense indeed (it's just 3 values, transferring them all at once is not expensive at all). Paolo -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
