[PATCH v8 08/11] KVM: TDX: Get/put PAMT pages when (un)mapping private memory

[Rick Edgecombe <rick.p.edgecombe@intel.com>]

From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>

Add Dynamic PAMT support to KVM's S-EPT MMU by "getting" a PAMT page when
adding guest memory (PAGE.ADD or PAGE.AUG), and "putting" the page when
removing guest memory (PAGE.REMOVE).

To access the per-vCPU PAMT caches without plumbing @vcpu throughout the
TDP MMU, begrudgingly use kvm_get_running_vcpu() to get the vCPU, and bug
the VM if KVM attempts to set an S-EPT leaf without an active vCPU.  KVM
only supports creating _new_ mappings in page (pre)fault paths, all of
which require an active vCPU.

The PAMT memory holds metadata for TDX protected memory. With Dynamic
PAMT, PAMT_4K is allocated on demand. The kernel supplies the TDX module
with a few pages that cover 2MB of host physical memory.

Releases are balanced via tdx_pamt_put(): every control-page free goes
through tdx_free_control_page(), and guest data pages are put directly on
the successful tdh_mem_page_remove() path and in the
tdx_mem_page_add/aug() error path.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Co-developed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
[rick: enhance log, reviewing, rebase, with help from AI tooling]
Co-developed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Tested-by: Hongyu Ning <hongyu.ning@linux.intel.com>
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
Reviewed-by: Tony Lindgren <tony.lindgren@linux.intel.com>
Acked-by: Sohil Mehta <sohil.mehta@intel.com>
Acked-by: Sean Christopherson <seanjc@google.com>
---
v8:
 - Remove extra new line around *topup_external_cache() (Sean)
 - Make +1/-1 logic clearer by just coding the easily optimizable math.
   (Nikolay, Sean)
 - Do KVM_BUG_ON() on tdx_pamt_get() return. (Sean)

v7:
 - Don't do to_tdx() before NULL check for readability (Binbin, Sean)
 - Fixup tags (Sean)
 - Export tdx_supports_dynamic_pamt() since it's used in KVM here and no
   longer an static inline.
---
 arch/x86/include/asm/kvm-x86-ops.h |  1 +
 arch/x86/include/asm/kvm_host.h    |  1 +
 arch/x86/kvm/mmu/mmu.c             |  4 ++
 arch/x86/kvm/vmx/tdx.c             | 64 ++++++++++++++++++++++++++----
 arch/x86/kvm/vmx/tdx.h             |  2 +
 arch/x86/virt/vmx/tdx/tdx.c        |  1 +
 6 files changed, 65 insertions(+), 8 deletions(-)

diff --git a/arch/x86/include/asm/kvm-x86-ops.h b/arch/x86/include/asm/kvm-x86-ops.h
index 83dc5086138b3..588563dfe88d5 100644
--- a/arch/x86/include/asm/kvm-x86-ops.h
+++ b/arch/x86/include/asm/kvm-x86-ops.h
@@ -98,6 +98,7 @@ KVM_X86_OP_OPTIONAL_RET0(tdp_has_smep)
 KVM_X86_OP(load_mmu_pgd)
 KVM_X86_OP_OPTIONAL_RET0(set_external_spte)
 KVM_X86_OP_OPTIONAL(free_external_spt)
+KVM_X86_OP_OPTIONAL_RET0(topup_external_cache)
 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 5f6c1ce9673b7..0dbf6564dd0c5 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1922,6 +1922,7 @@ struct kvm_x86_ops {
 	/* Update external page tables for page table about to be freed. */
 	void (*free_external_spt)(struct kvm *kvm, struct kvm_mmu_page *sp);
 
+	int (*topup_external_cache)(struct kvm_vcpu *vcpu, int min_nr_spts);
 
 	bool (*has_wbinvd_exit)(void);
 
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 234d0a95abf53..6dab99654f170 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -614,6 +614,10 @@ static int mmu_topup_memory_caches(struct kvm_vcpu *vcpu, bool maybe_indirect)
 					       PT64_ROOT_MAX_LEVEL);
 		if (r)
 			return r;
+
+		r = kvm_x86_call(topup_external_cache)(vcpu, PT64_ROOT_MAX_LEVEL);
+		if (r)
+			return r;
 	}
 	r = kvm_mmu_topup_memory_cache(&vcpu->arch.mmu_shadow_page_cache,
 				       PT64_ROOT_MAX_LEVEL);
diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
index 957078e8656ef..09573112540e9 100644
--- a/arch/x86/kvm/vmx/tdx.c
+++ b/arch/x86/kvm/vmx/tdx.c
@@ -681,6 +681,8 @@ int tdx_vcpu_create(struct kvm_vcpu *vcpu)
 	if (!irqchip_split(vcpu->kvm))
 		return -EINVAL;
 
+	tdx_init_pamt_cache(&tdx->pamt_cache);
+
 	fpstate_set_confidential(&vcpu->arch.guest_fpu);
 	vcpu->arch.apic->guest_apic_protected = true;
 	INIT_LIST_HEAD(&tdx->vt.pi_wakeup_list);
@@ -866,6 +868,8 @@ void tdx_vcpu_free(struct kvm_vcpu *vcpu)
 	struct vcpu_tdx *tdx = to_tdx(vcpu);
 	int i;
 
+	tdx_free_pamt_cache(&tdx->pamt_cache);
+
 	if (vcpu->cpu != -1) {
 		KVM_BUG_ON(tdx->state == VCPU_TD_STATE_INITIALIZED, vcpu->kvm);
 		tdx_flush_vp_on_cpu(vcpu);
@@ -1621,6 +1625,17 @@ void tdx_load_mmu_pgd(struct kvm_vcpu *vcpu, hpa_t root_hpa, int pgd_level)
 	td_vmcs_write64(to_tdx(vcpu), SHARED_EPT_POINTER, root_hpa);
 }
 
+static int tdx_topup_external_pamt_cache(struct kvm_vcpu *vcpu, int min_nr_spts)
+{
+	/*
+	 * Minus one page to exclude the root SPT, but plus one page for a
+	 * possible 4KB private mapping.
+	 */
+	min_nr_spts += -1 + 1;
+
+	return tdx_topup_pamt_cache(&to_tdx(vcpu)->pamt_cache, min_nr_spts);
+}
+
 static int tdx_mem_page_add(struct kvm *kvm, gfn_t gfn, enum pg_level level,
 			    kvm_pfn_t pfn)
 {
@@ -1679,16 +1694,28 @@ static struct page *tdx_spte_to_sept_pt(struct kvm *kvm, gfn_t gfn,
 static int tdx_sept_map_nonleaf_spte(struct kvm *kvm, gfn_t gfn,
 				     enum pg_level level, u64 new_spte)
 {
+	struct kvm_vcpu *vcpu = kvm_get_running_vcpu();
 	gpa_t gpa = gfn_to_gpa(gfn);
 	u64 err, entry, level_state;
 	struct page *sept_pt;
+	int ret;
+
+	if (KVM_BUG_ON(!vcpu, kvm))
+		return -EIO;
 
 	sept_pt = tdx_spte_to_sept_pt(kvm, gfn, new_spte, level);
 	if (!sept_pt)
 		return -EIO;
 
+	ret = tdx_pamt_get(page_to_pfn(sept_pt), &to_tdx(vcpu)->pamt_cache);
+	if (KVM_BUG_ON(ret, kvm))
+		return ret;
+
 	err = tdh_mem_sept_add(&to_kvm_tdx(kvm)->td, gpa, level, sept_pt,
 			       &entry, &level_state);
+	if (err)
+		tdx_pamt_put(page_to_pfn(sept_pt));
+
 	if (unlikely(tdx_operand_busy(err)))
 		return -EBUSY;
 
@@ -1701,8 +1728,13 @@ static int tdx_sept_map_nonleaf_spte(struct kvm *kvm, gfn_t gfn,
 static int tdx_sept_map_leaf_spte(struct kvm *kvm, gfn_t gfn, enum pg_level level,
 				  u64 new_spte)
 {
+	struct kvm_vcpu *vcpu = kvm_get_running_vcpu();
 	struct kvm_tdx *kvm_tdx = to_kvm_tdx(kvm);
 	kvm_pfn_t pfn = spte_to_pfn(new_spte);
+	int ret;
+
+	if (KVM_BUG_ON(!vcpu, kvm))
+		return -EIO;
 
 	/* TODO: handle large pages. */
 	if (KVM_BUG_ON(level != PG_LEVEL_4K, kvm))
@@ -1710,6 +1742,10 @@ static int tdx_sept_map_leaf_spte(struct kvm *kvm, gfn_t gfn, enum pg_level leve
 
 	WARN_ON_ONCE((new_spte & VMX_EPT_RWX_MASK) != VMX_EPT_RWX_MASK);
 
+	ret = tdx_pamt_get(pfn, &to_tdx(vcpu)->pamt_cache);
+	if (KVM_BUG_ON(ret, kvm))
+		return ret;
+
 	/*
 	 * Ensure pre_fault_allowed is read by kvm_arch_vcpu_pre_fault_memory()
 	 * before kvm_tdx->state.  Userspace must not be allowed to pre-fault
@@ -1722,10 +1758,15 @@ static int tdx_sept_map_leaf_spte(struct kvm *kvm, gfn_t gfn, enum pg_level leve
 	 * If the TD isn't finalized/runnable, then userspace is initializing
 	 * the VM image via KVM_TDX_INIT_MEM_REGION; ADD the page to the TD.
 	 */
-	if (unlikely(kvm_tdx->state != TD_STATE_RUNNABLE))
-		return tdx_mem_page_add(kvm, gfn, level, pfn);
+	if (likely(kvm_tdx->state == TD_STATE_RUNNABLE))
+		ret = tdx_mem_page_aug(kvm, gfn, level, pfn);
+	else
+		ret = tdx_mem_page_add(kvm, gfn, level, pfn);
 
-	return tdx_mem_page_aug(kvm, gfn, level, pfn);
+	if (ret)
+		tdx_pamt_put(pfn);
+
+	return ret;
 }
 
 /*
@@ -1822,6 +1863,7 @@ static int tdx_sept_remove_leaf_spte(struct kvm *kvm, gfn_t gfn,
 		return -EIO;
 
 	tdx_quirk_reset_paddr(PFN_PHYS(pfn), PAGE_SIZE);
+	tdx_pamt_put(pfn);
 	return 0;
 }
 
@@ -1865,6 +1907,8 @@ static int tdx_sept_set_private_spte(struct kvm *kvm, gfn_t gfn, u64 old_spte,
  */
 static void tdx_sept_free_private_spt(struct kvm *kvm, struct kvm_mmu_page *sp)
 {
+	struct page *sept_pt = virt_to_page(sp->external_spt);
+
 	/*
 	 * KVM doesn't (yet) zap page table pages in mirror page table while
 	 * TD is active, though guest pages mapped in mirror page table could be
@@ -1878,15 +1922,15 @@ static void tdx_sept_free_private_spt(struct kvm *kvm, struct kvm_mmu_page *sp)
 	 * the page to prevent the kernel from accessing the encrypted page.
 	 */
 	if (KVM_BUG_ON(is_hkid_assigned(to_kvm_tdx(kvm)), kvm) ||
-	    tdx_reclaim_page(virt_to_page(sp->external_spt)))
+	    tdx_reclaim_page(sept_pt))
 		goto out;
 
 	/*
-	 * Immediately free the S-EPT page because RCU-time free is unnecessary
-	 * after TDH.PHYMEM.PAGE.RECLAIM ensures there are no outstanding
-	 * readers.
+	 * Immediately free the S-EPT page as the TDX subsystem doesn't support
+	 * freeing pages from RCU callbacks, and more importantly because
+	 * TDH.PHYMEM.PAGE.RECLAIM ensures there are no outstanding readers.
 	 */
-	free_page((unsigned long)sp->external_spt);
+	tdx_free_control_page(sept_pt);
 out:
 	sp->external_spt = NULL;
 }
@@ -3486,6 +3530,10 @@ int __init tdx_hardware_setup(void)
 
 	vt_x86_ops.set_external_spte = tdx_sept_set_private_spte;
 	vt_x86_ops.free_external_spt = tdx_sept_free_private_spt;
+
+	if (tdx_supports_dynamic_pamt(tdx_sysinfo))
+		vt_x86_ops.topup_external_cache = tdx_topup_external_pamt_cache;
+
 	vt_x86_ops.protected_apic_has_interrupt = tdx_protected_apic_has_interrupt;
 	return 0;
 
diff --git a/arch/x86/kvm/vmx/tdx.h b/arch/x86/kvm/vmx/tdx.h
index ac8323a68b163..fd368e3ee060b 100644
--- a/arch/x86/kvm/vmx/tdx.h
+++ b/arch/x86/kvm/vmx/tdx.h
@@ -72,6 +72,8 @@ struct vcpu_tdx {
 
 	u64 map_gpa_next;
 	u64 map_gpa_end;
+
+	struct tdx_pamt_cache pamt_cache;
 };
 
 void tdh_vp_rd_failed(struct vcpu_tdx *tdx, char *uclass, u32 field, u64 err);
diff --git a/arch/x86/virt/vmx/tdx/tdx.c b/arch/x86/virt/vmx/tdx/tdx.c
index 4d90d1a555135..7fd427174b8b6 100644
--- a/arch/x86/virt/vmx/tdx/tdx.c
+++ b/arch/x86/virt/vmx/tdx/tdx.c
@@ -2044,6 +2044,7 @@ bool tdx_supports_dynamic_pamt(const struct tdx_sys_info *sysinfo)
 	/* To be enabled when kernel is ready. */
 	return false;
 }
+EXPORT_SYMBOL_FOR_KVM(tdx_supports_dynamic_pamt);
 
 static struct page *tdx_alloc_page_pamt_cache(struct tdx_pamt_cache *cache)
 {
-- 
2.54.0