On Wed, Aug 17, 2022 at 3:05 AM Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote: > > On 8/16/22 01:01, David Matlack wrote: > > Delete the module parameter tdp_mmu and force KVM to always use the TDP > > MMU when TDP hardware support is enabled. > > > > 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. > > > > Dropping the ability to disable the TDP MMU reduces the number of > > possible configurations that need to be tested to validate KVM (i.e. no > > need to test with tdp_mmu=N), and simplifies the code. > > The snag is that the shadow MMU is only supported on 64-bit systems; > testing tdp_mmu=0 is not a full replacement for booting up a 32-bit > distro, but it's easier (I do 32-bit testing only with nested virt). Ah, I did forget about 32-bit systems :(. Do Intel or AMD CPUs support TDP in 32-bit mode? > Personally I'd have no problem restricting KVM to x86-64 but I know > people would complain. As a middle-ground we could stop supporting TDP on 32-bit systems. 32-bit KVM would continue working but just with shadow paging. > > What about making the tdp_mmu module parameter read-only, so that at > least kvm->arch.tdp_mmu_enabled can be replaced by a global variable? > > Paolo >
