On Tuesday, December 21, 2021 4:45 PM, Paolo Bonzini wrote: > On 12/21/21 03:45, Wang, Wei W wrote: > >> KVM_GET_XSAVE2 and KVM_SET_XSAVE respectively write and read as > many > >> bytes as are returned by KVM_CHECK_EXTENSION(KVM_CAP_XSAVE2), > when > >> invoked on the vm file descriptor. Currently, > >> KVM_CHECK_EXTENSION(KVM_CAP_XSAVE2) will only return a value that is > >> greater than 4096 bytes if any dynamic features have been enabled > >> with ``arch_prctl()``; this however may change in the future. > > Would this make people think that > KVM_CHECK_EXTENSION(KVM_CAP_XSAVE2) > > doesn’t return the value (i.e. return 0) if it is smaller than 4096? > > (i.e. KVM_GET_XSAVE2 doesn't work with size < 4096, which isn’t true) > > > > I plan to just reword a bit: > > Currently, KVM_CHECK_EXTENSION(KVM_CAP_XSAVE2) will only return a > size > > value, and the value is greater than 4096 bytes if any dynamic > > features have been enabled with ``arch_prctl()``. More types of values could > be returned in the future. > > Next refinement: > > The size value returned by KVM_CHECK_EXTENSION(KVM_CAP_XSAVE2) will > always be at least 4096. Currently, it is only greater than 4096 if a dynamic > feature has been enabled with ``arch_prctl()``, but this may change in the > future. > (I'm not sure if the first sentence is true in the code, but if not it is a bug that > has to be fixed :)). For the implementation, KVM_CHECK_EXTENSION(KVM_CAP_XSAVE2) always return kvm->vcpus[0]->arch.guest_fpu.uabi_size. Do you want to change it to below? If (kvm->vcpus[0]->arch.guest_fpu.uabi_size < 4096) return 0; else return kvm->vcpus[0]->arch.guest_fpu.uabi_size; If the size is less than 4096 (e.g. no dynamic xfeatures enabled), userspace should use the old KVM_GET_XSAVE (instead of KVM_GET_XSAVE2)? (KVM_GET_XSAVE2 supports to work with size less than 4096, so I think this isn't necessary) Thanks, Wei
