Re: [PATCH v2 07/20] kvm: x86/mmu: Support zapping SPTEs in the TDP MMU

[Yu Zhang]

On Wed, Oct 21, 2020 at 08:00:47PM +0200, Paolo Bonzini wrote:
> On 21/10/20 19:24, Yu Zhang wrote:
> > On Wed, Oct 21, 2020 at 07:20:15PM +0200, Paolo Bonzini wrote:
> >> On 21/10/20 17:02, Yu Zhang wrote:
> >>>>  void kvm_tdp_mmu_free_root(struct kvm *kvm, struct kvm_mmu_page *root)
> >>>>  {
> >>>> +	gfn_t max_gfn = 1ULL << (boot_cpu_data.x86_phys_bits - PAGE_SHIFT);
> >>>> +
> >>> boot_cpu_data.x86_phys_bits is the host address width. Value of the guest's
> >>> may vary. So maybe we should just traverse the memslots and zap the gfn ranges
> >>> in each of them?
> >>>
> >>
> >> It must be smaller than the host value for two-dimensional paging, though.
> > 
> > Yes. And using boot_cpu_data.x86_phys_bits works, but won't it be somewhat
> > overkilling? E.g. for a host with 46 bits and a guest with 39 bits width?
> 
> It would go quickly through extra memory space because the PML4E entries
> above the first would be empty.  So it's just 511 comparisons.
> 

Oh, yes. The overhead seems not as big as I assumed. :)

Yu
> Paolo
>