Re: [PATCH v19 062/130] KVM: x86/tdp_mmu: Support TDX private mapping for TDP MMU

Isaku Yamahata
2024-03-25T20:01:23Z

On Sat, Mar 23, 2024 at 11:39:07PM +0000,
"Edgecombe, Rick P" <rick.p.edgecombe@intel.com> wrote:

> On Mon, 2024-02-26 at 00:26 -0800, isaku.yamahata@intel.com wrote:
> > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> > index efd3fda1c177..bc0767c884f7 100644
> > --- a/arch/x86/include/asm/kvm_host.h
> > +++ b/arch/x86/include/asm/kvm_host.h
> > @@ -468,6 +468,7 @@ struct kvm_mmu {
> >         int (*sync_spte)(struct kvm_vcpu *vcpu,
> >                          struct kvm_mmu_page *sp, int i);
> >         struct kvm_mmu_root_info root;
> > +       hpa_t private_root_hpa;
>
> Per the conversation about consistent naming between private, shared and mirror: I wonder if this
> should be named these with mirror instead of private. Like:
> hpa_t mirror_root_hpa;
>
> Since the actual private root is not tracked by KVM.

It's mirrored only without direct associated Secure EPT page. The association
is implicit, a part of TDCS pages.

> > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > index 30c86e858ae4..0e0321ad9ca2 100644
> > --- a/arch/x86/kvm/mmu/mmu.c
> > +++ b/arch/x86/kvm/mmu/mmu.c
> > @@ -3717,7 +3717,12 @@ static int mmu_alloc_direct_roots(struct kvm_vcpu *vcpu)
> >                 goto out_unlock;
> >
> >         if (tdp_mmu_enabled) {
> > -               root = kvm_tdp_mmu_get_vcpu_root_hpa(vcpu);
> > +               if (kvm_gfn_shared_mask(vcpu->kvm) &&
> > +                   !VALID_PAGE(mmu->private_root_hpa)) {
> > +                       root = kvm_tdp_mmu_get_vcpu_root_hpa(vcpu, true);
> > +                       mmu->private_root_hpa = root;
> > +               }
> > +               root = kvm_tdp_mmu_get_vcpu_root_hpa(vcpu, false);
> >                 mmu->root.hpa = root;
>
> This has changed now, due to rebase on,
> https://lore.kernel.org/lkml/20240111020048.844847-1-seanjc@google.com/
>
> ...to this:
> - if (tdp_mmu_enabled)
> - return kvm_tdp_mmu_alloc_root(vcpu);
> + if (tdp_mmu_enabled) {
> + if (kvm_gfn_shared_mask(vcpu->kvm) &&
> + !VALID_PAGE(mmu->private_root_hpa)) {
> + r = kvm_tdp_mmu_alloc_root(vcpu, true);
> + if (r)
> + return r;
> + }
> + return kvm_tdp_mmu_alloc_root(vcpu, false);
> + }
>
> I don't see why the !VALID_PAGE(mmu->private_root_hpa) check is needed.
> kvm_tdp_mmu_get_vcpu_root_hpa() already has logic to prevent allocating multiple roots with the same
> role.

Historically we needed it. We don't need it now. We can drop it.

> Also, kvm_tdp_mmu_alloc_root() never returns non-zero, even though mmu_alloc_direct_roots() does.
> Probably today when there is one caller it makes mmu_alloc_direct_roots() cleaner to just have it
> return the always zero value from kvm_tdp_mmu_alloc_root(). Now that there are two calls, I think we
> should refactor kvm_tdp_mmu_alloc_root() to return void, and have kvm_tdp_mmu_alloc_root() return 0
> manually in this case.
>
> Or maybe instead change it back to returning an hpa_t and then kvm_tdp_mmu_alloc_root() can lose the
> "if (private)" logic at the end too.

Probably we can make void kvm_tdp_mmu_alloc_root() instead of returning always
zero as clean up.

> >         } else if (shadow_root_level >= PT64_ROOT_4LEVEL) {
> >                 root = mmu_alloc_root(vcpu, 0, 0, shadow_root_level);
> > @@ -4627,7 +4632,7 @@ int kvm_tdp_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
> >         if (kvm_mmu_honors_guest_mtrrs(vcpu->kvm)) {
> >                 for ( ; fault->max_level > PG_LEVEL_4K; --fault->max_level) {
> >                         int page_num = KVM_PAGES_PER_HPAGE(fault->max_level);
> > -                       gfn_t base = gfn_round_for_level(fault->gfn,
> > +                       gfn_t base = gfn_round_for_level(gpa_to_gfn(fault->addr),
> >                                                          fault->max_level);
> >
> >                         if (kvm_mtrr_check_gfn_range_consistency(vcpu, base, page_num))
> > @@ -4662,6 +4667,7 @@ int kvm_mmu_map_tdp_page(struct kvm_vcpu *vcpu, gpa_t gpa, u64 error_code,
> >         };
> >
> >         WARN_ON_ONCE(!vcpu->arch.mmu->root_role.direct);
> > +       fault.gfn = gpa_to_gfn(fault.addr) & ~kvm_gfn_shared_mask(vcpu->kvm);
> >         fault.slot = kvm_vcpu_gfn_to_memslot(vcpu, fault.gfn);
> >
> >         r = mmu_topup_memory_caches(vcpu, false);
> > @@ -6166,6 +6172,7 @@ static int __kvm_mmu_create(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu)
> >
> >         mmu->root.hpa = INVALID_PAGE;
> >         mmu->root.pgd = 0;
> > +       mmu->private_root_hpa = INVALID_PAGE;
> >         for (i = 0; i < KVM_MMU_NUM_PREV_ROOTS; i++)
> >                 mmu->prev_roots[i] = KVM_MMU_ROOT_INFO_INVALID;
> >
> > @@ -7211,6 +7218,12 @@ int kvm_mmu_vendor_module_init(void)
> >  void kvm_mmu_destroy(struct kvm_vcpu *vcpu)
> >  {
> >         kvm_mmu_unload(vcpu);
> > +       if (tdp_mmu_enabled) {
> > +               write_lock(&vcpu->kvm->mmu_lock);
> > +               mmu_free_root_page(vcpu->kvm, &vcpu->arch.mmu->private_root_hpa,
> > +                               NULL);
> > +               write_unlock(&vcpu->kvm->mmu_lock);
>
> What is the reason for the special treatment of private_root_hpa here? The rest of the roots are
> freed in kvm_mmu_unload(). I think it is because we don't want the mirror to get freed during
> kvm_mmu_reset_context()?

It reflects that we don't free Secure-EPT pages during runtime, and free them
when destroying the guest.

>
> Oof. For the sake of trying to justify the code, I'm trying to keep track of the pros and cons of
> treating the mirror/private root like a normal one with just a different role bit.
>
> The whole “list of roots” thing seems to date from the shadow paging, where there is is critical to
> keep multiple cached shared roots of different CPU modes of the same shadowed page tables. Today
> with non-nested TDP, AFAICT, the only different root is for SMM. I guess since the machinery for
> managing multiple roots in a list already exists it makes sense to use it for both.
>
> For TDX there are also only two, but the difference is, things need to be done in special ways for
> the two roots. You end up with a bunch of loops (for_each_*tdp_mmu_root(), etc) that essentially
> process a list of two different roots, but with inner logic tortured to work for the peculiarities
> of both private and shared. An easier to read alternative could be to open code both cases.
>
> I guess the major benefit is to keep one set of logic for shadow paging, normal TDP and TDX, but it
> makes the logic a bit difficult to follow for TDX compared to looking at it from the normal guest
> perspective. So I wonder if making special versions of the TDX root traversing operations might make
> the code a little easier to follow. I’m not advocating for it at this point, just still working on
> an opinion. Is there any history around this design point?

The original desire to keep the modification contained, and not introduce a
function for population and zap. With the open coding, do you want something
like the followings? We can try it and compare the outcome.

For zapping
if (private) {
__for_each_tdp_mmu_root_yield_safe_private()
private case
} else {
__for_each_tdp_mmu_root_yield_safe()
shared case
}

For fault,
kvm_tdp_mmu_map()
if (private) {
tdp_mmu_for_each_pte_private(iter, mmu, raw_gfn, raw_gfn + 1)
private case
} else {
tdp_mmu_for_each_pte_private(iter, mmu, raw_gfn, raw_gfn + 1)
shared case
}

--
Isaku Yamahata <isaku.yamahata@intel.com>