Re: [PATCH v2 0/4] KVM: x86: Partially allow KVM_SET_CPUID{,2} after KVM_RUN for CPU hotplug

[Paolo Bonzini]

On 1/18/22 17:53, Sean Christopherson wrote:
> > and I think we should redo all or most of kvm_update_cpuid_runtime
> > the same way.
> Please no.  xstate_required_size() requires multiple host CPUID calls, and glibc
> does CPUID.0xD.0x0 and CPUID.0xD.0x1 as part of its initialization, i.e. launching
> a new userspace process in the guest will see additional performance overhread due
> to KVM dynamically computing the XSAVE size instead of caching it based on vCPU
> state.  Nested virtualization would be especially painful as every one of those
> "host" CPUID invocations will trigger and exit from L1=>L0.

Agreed, but all of the required data is by Linux cached in 
xstate_offsets, xstate_sizes and xstate_comp_offsets; moving 
xstate_required_size to xstate.c and skipping the host CPUID would 
probably be a good idea nevertheless.

Paolo