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Date: Wed, 15 May 2024 18:48:03 -0700
From: Isaku Yamahata <isaku.yamahata@...el.com>
To: "Huang, Kai" <kai.huang@...el.com>
Cc: Rick Edgecombe <rick.p.edgecombe@...el.com>, pbonzini@...hat.com,
seanjc@...gle.com, kvm@...r.kernel.org,
linux-kernel@...r.kernel.org, isaku.yamahata@...il.com,
erdemaktas@...gle.com, sagis@...gle.com, yan.y.zhao@...el.com,
dmatlack@...gle.com
Subject: Re: [PATCH 10/16] KVM: x86/tdp_mmu: Support TDX private mapping for
TDP MMU
On Thu, May 16, 2024 at 12:52:32PM +1200,
"Huang, Kai" <kai.huang@...el.com> wrote:
> On 15/05/2024 12:59 pm, Rick Edgecombe wrote:
> > From: Isaku Yamahata <isaku.yamahata@...el.com>
> >
> > Allocate mirrored page table for the private page table and implement MMU
> > hooks to operate on the private page table.
> >
> > To handle page fault to a private GPA, KVM walks the mirrored page table in
> > unencrypted memory and then uses MMU hooks in kvm_x86_ops to propagate
> > changes from the mirrored page table to private page table.
> >
> > private KVM page fault |
> > | |
> > V |
> > private GPA | CPU protected EPTP
> > | | |
> > V | V
> > mirrored PT root | private PT root
> > | | |
> > V | V
> > mirrored PT --hook to propagate-->private PT
> > | | |
> > \--------------------+------\ |
> > | | |
> > | V V
> > | private guest page
> > |
> > |
> > non-encrypted memory | encrypted memory
> > |
> >
> > PT: page table
> > Private PT: the CPU uses it, but it is invisible to KVM. TDX module manages
> > this table to map private guest pages.
> > Mirrored PT:It is visible to KVM, but the CPU doesn't use it. KVM uses it
> > to propagate PT change to the actual private PT.
> >
> > SPTEs in mirrored page table (refer to them as mirrored SPTEs hereafter)
> > can be modified atomically with mmu_lock held for read, however, the MMU
> > hooks to private page table are not atomical operations.
> >
> > To address it, a special REMOVED_SPTE is introduced and below sequence is
> > used when mirrored SPTEs are updated atomically.
> >
> > 1. Mirrored SPTE is first atomically written to REMOVED_SPTE.
> > 2. The successful updater of the mirrored SPTE in step 1 proceeds with the
> > following steps.
> > 3. Invoke MMU hooks to modify private page table with the target value.
> > 4. (a) On hook succeeds, update mirrored SPTE to target value.
> > (b) On hook failure, restore mirrored SPTE to original value.
> >
> > KVM TDP MMU ensures other threads will not overrite REMOVED_SPTE.
> >
> > This sequence also applies when SPTEs are atomiclly updated from
> > non-present to present in order to prevent potential conflicts when
> > multiple vCPUs attempt to set private SPTEs to a different page size
> > simultaneously, though 4K page size is only supported for private page
> > table currently.
> >
> > 2M page support can be done in future patches.
> >
> > Signed-off-by: Isaku Yamahata <isaku.yamahata@...el.com>
> > Co-developed-by: Kai Huang <kai.huang@...el.com>
> > Signed-off-by: Kai Huang <kai.huang@...el.com>
> > Co-developed-by: Yan Zhao <yan.y.zhao@...el.com>
> > Signed-off-by: Yan Zhao <yan.y.zhao@...el.com>
> > Co-developed-by: Rick Edgecombe <rick.p.edgecombe@...el.com>
> > Signed-off-by: Rick Edgecombe <rick.p.edgecombe@...el.com>
> > ---
> > TDX MMU Part 1:
> > - Remove unnecessary gfn, access twist in
> > tdp_mmu_map_handle_target_level(). (Chao Gao)
> > - Open code call to kvm_mmu_alloc_private_spt() instead oCf doing it in
> > tdp_mmu_alloc_sp()
> > - Update comment in set_private_spte_present() (Yan)
> > - Open code call to kvm_mmu_init_private_spt() (Yan)
> > - Add comments on TDX MMU hooks (Yan)
> > - Fix various whitespace alignment (Yan)
> > - Remove pointless warnings and conditionals in
> > handle_removed_private_spte() (Yan)
> > - Remove redundant lockdep assert in tdp_mmu_set_spte() (Yan)
> > - Remove incorrect comment in handle_changed_spte() (Yan)
> > - Remove unneeded kvm_pfn_to_refcounted_page() and
> > is_error_noslot_pfn() check in kvm_tdp_mmu_map() (Yan)
> > - Do kvm_gfn_for_root() branchless (Rick)
> > - Update kvm_tdp_mmu_alloc_root() callers to not check error code (Rick)
> > - Add comment for stripping shared bit for fault.gfn (Chao)
> >
> > v19:
> > - drop CONFIG_KVM_MMU_PRIVATE
> >
> > v18:
> > - Rename freezed => frozen
> >
> > v14 -> v15:
> > - Refined is_private condition check in kvm_tdp_mmu_map().
> > Add kvm_gfn_shared_mask() check.
> > - catch up for struct kvm_range change
> > ---
> > arch/x86/include/asm/kvm-x86-ops.h | 5 +
> > arch/x86/include/asm/kvm_host.h | 25 +++
> > arch/x86/kvm/mmu/mmu.c | 13 +-
> > arch/x86/kvm/mmu/mmu_internal.h | 19 +-
> > arch/x86/kvm/mmu/tdp_iter.h | 2 +-
> > arch/x86/kvm/mmu/tdp_mmu.c | 269 +++++++++++++++++++++++++----
> > arch/x86/kvm/mmu/tdp_mmu.h | 2 +-
> > 7 files changed, 293 insertions(+), 42 deletions(-)
> >
> > diff --git a/arch/x86/include/asm/kvm-x86-ops.h b/arch/x86/include/asm/kvm-x86-ops.h
> > index 566d19b02483..d13cb4b8fce6 100644
> > --- a/arch/x86/include/asm/kvm-x86-ops.h
> > +++ b/arch/x86/include/asm/kvm-x86-ops.h
> > @@ -95,6 +95,11 @@ KVM_X86_OP_OPTIONAL_RET0(set_tss_addr)
> > KVM_X86_OP_OPTIONAL_RET0(set_identity_map_addr)
> > KVM_X86_OP_OPTIONAL_RET0(get_mt_mask)
> > KVM_X86_OP(load_mmu_pgd)
> > +KVM_X86_OP_OPTIONAL(link_private_spt)
> > +KVM_X86_OP_OPTIONAL(free_private_spt)
> > +KVM_X86_OP_OPTIONAL(set_private_spte)
> > +KVM_X86_OP_OPTIONAL(remove_private_spte)
> > +KVM_X86_OP_OPTIONAL(zap_private_spte)
> > KVM_X86_OP(has_wbinvd_exit)
> > KVM_X86_OP(get_l2_tsc_offset)
> > KVM_X86_OP(get_l2_tsc_multiplier)
> > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> > index d010ca5c7f44..20fa8fa58692 100644
> > --- a/arch/x86/include/asm/kvm_host.h
> > +++ b/arch/x86/include/asm/kvm_host.h
> > @@ -470,6 +470,7 @@ 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;
>
> Should we have
>
> struct kvm_mmu_root_info private_root;
>
> instead?
Yes. And the private root allocation can be pushed down into TDP MMU.
> > union kvm_cpu_role cpu_role;
> > union kvm_mmu_page_role root_role;
> > @@ -1747,6 +1748,30 @@ struct kvm_x86_ops {
> > void (*load_mmu_pgd)(struct kvm_vcpu *vcpu, hpa_t root_hpa,
> > int root_level);
> > + /* Add a page as page table page into private page table */
> > + int (*link_private_spt)(struct kvm *kvm, gfn_t gfn, enum pg_level level,
> > + void *private_spt);
> > + /*
> > + * Free a page table page of private page table.
> > + * Only expected to be called when guest is not active, specifically
> > + * during VM destruction phase.
> > + */
> > + int (*free_private_spt)(struct kvm *kvm, gfn_t gfn, enum pg_level level,
> > + void *private_spt);
> > +
> > + /* Add a guest private page into private page table */
> > + int (*set_private_spte)(struct kvm *kvm, gfn_t gfn, enum pg_level level,
> > + kvm_pfn_t pfn);
> > +
> > + /* Remove a guest private page from private page table*/
> > + int (*remove_private_spte)(struct kvm *kvm, gfn_t gfn, enum pg_level level,
> > + kvm_pfn_t pfn);
> > + /*
> > + * Keep a guest private page mapped in private page table, but clear its
> > + * present bit
> > + */
> > + int (*zap_private_spte)(struct kvm *kvm, gfn_t gfn, enum pg_level level);
> > +
> > bool (*has_wbinvd_exit)(void);
> > u64 (*get_l2_tsc_offset)(struct kvm_vcpu *vcpu);
> > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > index 76f92cb37a96..2506d6277818 100644
> > --- a/arch/x86/kvm/mmu/mmu.c
> > +++ b/arch/x86/kvm/mmu/mmu.c
> > @@ -3701,7 +3701,9 @@ static int mmu_alloc_direct_roots(struct kvm_vcpu *vcpu)
> > int r;
> > if (tdp_mmu_enabled) {
> > - kvm_tdp_mmu_alloc_root(vcpu);
> > + if (kvm_gfn_shared_mask(vcpu->kvm))
> > + kvm_tdp_mmu_alloc_root(vcpu, true);
>
> As mentioned in replies to other patches, I kinda prefer
>
> kvm->arch.has_mirrored_pt (or has_mirrored_private_pt)
>
> Or we have a helper
>
> kvm_has_mirrored_pt() / kvm_has_mirrored_private_pt()
>
> > + kvm_tdp_mmu_alloc_root(vcpu, false);
> > return 0;
> > }
> > @@ -4685,7 +4687,7 @@ 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);
>
> I thought by reaching here the shared bit has already been stripped away by
> the caller?
>
> It doesn't make a lot sense to still have it here, given we have a universal
> KVM-defined PFERR_PRIVATE_ACCESS flag:
>
> https://lore.kernel.org/kvm/20240507155817.3951344-2-pbonzini@redhat.com/T/#mb30987f31b431771b42dfa64dcaa2efbc10ada5e
>
> IMHO we should just strip the shared bit in the TDX variant of
> handle_ept_violation(), and pass the PFERR_PRIVATE_ACCESS (when GPA doesn't
> hvae shared bit) to the common fault handler so it can correctly set
> fault->is_private to true.
Yes, this part should be dropped. Because we will have vCPUID.MTRR=0 for TDX in
long term, we can make kvm_mmu_honors_guest_mtrrs() always false. Maybe
kvm->arch.disbled_mtrr or guest_cpuid_has(vcpu, X86_FEATURE_MTRR) = false. We
will enforce vcpu.CPUID.MTRR=false.
Guest MTRR=0 support can be independently addressed.
> > if (kvm_mtrr_check_gfn_range_consistency(vcpu, base, page_num))
> > @@ -6245,6 +6247,7 @@ 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;
> > @@ -7263,6 +7266,12 @@ int kvm_mmu_vendor_module_init(void)
> > void kvm_mmu_destroy(struct kvm_vcpu *vcpu)
> > {
> > kvm_mmu_unload(vcpu);
> > + if (tdp_mmu_enabled) {
> > + read_lock(&vcpu->kvm->mmu_lock);
> > + mmu_free_root_page(vcpu->kvm, &vcpu->arch.mmu->private_root_hpa,
> > + NULL);
> > + read_unlock(&vcpu->kvm->mmu_lock);
> > + }
>
> Hmm.. I don't quite like this, but sorry I kinda forgot why we need to to
> this here.
>
> Could you elaborate?
>
> Anyway, from common code's perspective, we need to have some clarification
> why we design to do it here.
This should be cleaned up. It can be pushed down into kvm_tdp_mmu_alloc_root().
void kvm_tdp_mmu_alloc_root(struct kvm_vcpu *vcpu)
allocate shared root
if (has_mirrort_pt)
allocate private root
> > free_mmu_pages(&vcpu->arch.root_mmu);
> > free_mmu_pages(&vcpu->arch.guest_mmu);
> > mmu_free_memory_caches(vcpu);
> > diff --git a/arch/x86/kvm/mmu/mmu_internal.h b/arch/x86/kvm/mmu/mmu_internal.h
> > index 0f1a9d733d9e..3a7fe9261e23 100644
> > --- a/arch/x86/kvm/mmu/mmu_internal.h
> > +++ b/arch/x86/kvm/mmu/mmu_internal.h
> > @@ -6,6 +6,8 @@
> > #include <linux/kvm_host.h>
> > #include <asm/kvm_host.h>
> > +#include "mmu.h"
> > +
> > #ifdef CONFIG_KVM_PROVE_MMU
> > #define KVM_MMU_WARN_ON(x) WARN_ON_ONCE(x)
> > #else
> > @@ -178,6 +180,16 @@ static inline void kvm_mmu_alloc_private_spt(struct kvm_vcpu *vcpu, struct kvm_m
> > sp->private_spt = kvm_mmu_memory_cache_alloc(&vcpu->arch.mmu_private_spt_cache);
> > }
> > +static inline gfn_t kvm_gfn_for_root(struct kvm *kvm, struct kvm_mmu_page *root,
> > + gfn_t gfn)
> > +{
> > + gfn_t gfn_for_root = kvm_gfn_to_private(kvm, gfn);
> > +
> > + /* Set shared bit if not private */
> > + gfn_for_root |= -(gfn_t)!is_private_sp(root) & kvm_gfn_shared_mask(kvm);
> > + return gfn_for_root;
> > +}
> > +
> > static inline bool kvm_mmu_page_ad_need_write_protect(struct kvm_mmu_page *sp)
> > {
> > /*
> > @@ -348,7 +360,12 @@ static inline int __kvm_mmu_do_page_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gp
> > int r;
> > if (vcpu->arch.mmu->root_role.direct) {
> > - fault.gfn = fault.addr >> PAGE_SHIFT;
> > + /*
> > + * Things like memslots don't understand the concept of a shared
> > + * bit. Strip it so that the GFN can be used like normal, and the
> > + * fault.addr can be used when the shared bit is needed.
> > + */
> > + fault.gfn = gpa_to_gfn(fault.addr) & ~kvm_gfn_shared_mask(vcpu->kvm);
> > fault.slot = kvm_vcpu_gfn_to_memslot(vcpu, fault.gfn);
>
> Again, I don't think it's nessary for fault.gfn to still have the shared bit
> here?
>
> This kinda usage is pretty much the reason I want to get rid of
> kvm_gfn_shared_mask().
We are going to flags like has_mirrored_pt and we have root page table iterator
with types specified. I'll investigate how we can reduce (or eliminate)
those helper functions.
> > }
> > diff --git a/arch/x86/kvm/mmu/tdp_iter.h b/arch/x86/kvm/mmu/tdp_iter.h
> > index fae559559a80..8a64bcef9deb 100644
> > --- a/arch/x86/kvm/mmu/tdp_iter.h
> > +++ b/arch/x86/kvm/mmu/tdp_iter.h
> > @@ -91,7 +91,7 @@ struct tdp_iter {
> > tdp_ptep_t pt_path[PT64_ROOT_MAX_LEVEL];
> > /* A pointer to the current SPTE */
> > tdp_ptep_t sptep;
> > - /* The lowest GFN mapped by the current SPTE */
> > + /* The lowest GFN (shared bits included) mapped by the current SPTE */
> > gfn_t gfn;
>
> IMHO we need more clarification of this design.
>
> We at least needs to call out the TDX hardware uses the 'GFA + shared bit'
> when it walks the page table for shared mappings, so we must set up the
> mapping at the GPA with the shared bit.
>
> E.g, because TDX hardware uses separate root for shared/private mappings, I
> think it's a resonable opion for the TDX hardware to just use the actual GPA
> w/o shared bit when it walks the shared page table, and still report EPT
> violation with GPA with shared bit set.
>
> Such HW implementation is completely hidden from software, thus should be
> clarified in the changelog/comments.
Totally agree that it deserves for documentation. I would update the design
document of TDX TDP MMU to include it. This patch series doesn't include it,
though.
> > /* The level of the root page given to the iterator */
> > int root_level;
>
> [...]
>
> > for_each_tdp_pte_min_level(iter, root, PG_LEVEL_4K, start, end) {
> > @@ -1029,8 +1209,8 @@ static int tdp_mmu_map_handle_target_level(struct kvm_vcpu *vcpu,
> > new_spte = make_mmio_spte(vcpu, iter->gfn, ACC_ALL);
> > else
> > wrprot = make_spte(vcpu, sp, fault->slot, ACC_ALL, iter->gfn,
> > - fault->pfn, iter->old_spte, fault->prefetch, true,
> > - fault->map_writable, &new_spte);
> > + fault->pfn, iter->old_spte, fault->prefetch, true,
> > + fault->map_writable, &new_spte);
> > if (new_spte == iter->old_spte)
> > ret = RET_PF_SPURIOUS;
> > @@ -1108,6 +1288,8 @@ int kvm_tdp_mmu_map(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
> > struct kvm *kvm = vcpu->kvm;
> > struct tdp_iter iter;
> > struct kvm_mmu_page *sp;
> > + gfn_t raw_gfn;
> > + bool is_private = fault->is_private && kvm_gfn_shared_mask(kvm);
>
> Ditto. I wish we can have 'has_mirrored_private_pt'.
Which name do you prefer? has_mirrored_pt or has_mirrored_private_pt?
--
Isaku Yamahata <isaku.yamahata@...el.com>
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