Re: [PATCH 0/9] KVM: x86/mmu: Always enable the TDP MMU when TDP is enabled

["Huang, Kai" <kai.huang@intel.com>]

On Tue, 2022-08-16 at 09:30 -0700, David Matlack wrote:
> On Tue, Aug 16, 2022 at 1:17 AM Peter Zijlstra <peterz@infradead.org> wrote:
> > 
> > On Mon, Aug 15, 2022 at 04:01:01PM -0700, David Matlack wrote:
> > > Patch 1 deletes the module parameter tdp_mmu and forces KVM to always
> > > use the TDP MMU when TDP hardware support is enabled.  The rest of the
> > > patches are related cleanups that follow (although the kvm_faultin_pfn()
> > > cleanups at the end are only tangentially related at best).
> > > 
> > > The TDP MMU was introduced in 5.10 and has been enabled by default since
> > > 5.15. At this point there are no known functionality gaps between the
> > > TDP MMU and the shadow MMU, and the TDP MMU uses less memory and scales
> > > better with the number of vCPUs. In other words, there is no good reason
> > > to disable the TDP MMU.
> > 
> > Then how are you going to test the shadow mmu code -- which I assume is
> > still relevant for the platforms that don't have this hardware support
> > you speak of?
> 
> TDP hardware support can still be disabled with module parameters
> (kvm_intel.ept=N and kvm_amd.npt=N).
> 
> The tdp_mmu module parameter only controls whether KVM uses the TDP
> MMU or shadow MMU *when TDP hardware is enabled*.

With the tdp_mmu module parameter, when we develop some code, we can at least
easily test legacy MMU code (that it is still working) when *TDP hardware is
enabled* by turning the parameter off.  Or when there's some problem with TDP
MMU code, we can easily switch to use legacy MMU.

Do we want to lose those flexibilities?

-- 
Thanks,
-Kai