Re: [PATCH] KVM: nVMX: Don't expose TSC scaling to L1 when on Hyper-V

[Anirudh Rayabharam <anrayabh@linux.microsoft.com>]

On Wed, Jun 22, 2022 at 10:00:29AM +0200, Vitaly Kuznetsov wrote:
> Sean Christopherson <seanjc@google.com> writes:
> 
> > On Tue, Jun 14, 2022, Anirudh Rayabharam wrote:
> >> On Mon, Jun 13, 2022 at 04:57:49PM +0000, Sean Christopherson wrote:
> 
> ...
> 
> >> > 
> >> > Any reason not to use the already sanitized vmcs_config?  I can't think of any
> >> > reason why the nested path should blindly use the raw MSR values from hardware.
> >> 
> >> vmcs_config has the sanitized exec controls. But how do we construct MSR
> >> values using them?
> >
> > I was thinking we could use the sanitized controls for the allowed-1 bits, and then
> > take the required-1 bits from the CPU.  And then if we wanted to avoid the redundant
> > RDMSRs in a follow-up patch we could add required-1 fields to vmcs_config.
> >
> > Hastily constructed and compile-tested only, proceed with caution :-)
> 
> Independently from "[PATCH 00/11] KVM: VMX: Support TscScaling and
> EnclsExitingBitmap whith eVMCS" which is supposed to fix the particular
> TSC scaling issue, I like the idea to make nested_vmx_setup_ctls_msrs()
> use both allowed-1 and required-1 bits from vmcs_config. I'll pick up
> the suggested patch and try to construct something for required-1 bits.

I tried this patch today but it causes some regression which causes
/dev/kvm to be unavailable in L1. I didn't get a chance to look into it
closely but I am guessing it has something to do with the fact that
vmcs_config reflects the config that L0 chose to use rather than what is
available to use. So constructing allowed-1 MSR bits based on what bits
are set in exec controls maybe isn't correct.


Thanks!

	- Anirudh.