On Wed, Jul 09, 2025, Michael Roth wrote:
> On Thu, Jul 03, 2025 at 02:26:41PM +0800, Yan Zhao wrote:
> > Rather than invoking kvm_gmem_populate(), allow tdx_vcpu_init_mem_region()
> > to use open code to populate the initial memory region into the mirror page
> > table, and add the region to S-EPT.
> >
> > Background
> > ===
> > Sean initially suggested TDX to populate initial memory region in a 4-step
> > way [1]. Paolo refactored guest_memfd and introduced kvm_gmem_populate()
> > interface [2] to help TDX populate init memory region.
I wouldn't give my suggestion too much weight; I did qualify it with "Crazy idea."
after all :-)
> > tdx_vcpu_init_mem_region
> > guard(mutex)(&kvm->slots_lock)
> > kvm_gmem_populate
> > filemap_invalidate_lock(file->f_mapping)
> > __kvm_gmem_get_pfn //1. get private PFN
> > post_populate //tdx_gmem_post_populate
> > get_user_pages_fast //2. get source page
> > kvm_tdp_map_page //3. map private PFN to mirror root
> > tdh_mem_page_add //4. add private PFN to S-EPT and copy
> > source page to it.
> >
> > kvm_gmem_populate() helps TDX to "get private PFN" in step 1. Its file
> > invalidate lock also helps ensure the private PFN remains valid when
> > tdh_mem_page_add() is invoked in TDX's post_populate hook.
> >
> > Though TDX does not need the folio prepration code, kvm_gmem_populate()
> > helps on sharing common code between SEV-SNP and TDX.
> >
> > Problem
> > ===
> > (1)
> > In Michael's series "KVM: gmem: 2MB THP support and preparedness tracking
> > changes" [4], kvm_gmem_get_pfn() was modified to rely on the filemap
> > invalidation lock for protecting its preparedness tracking. Similarly, the
> > in-place conversion version of guest_memfd series by Ackerly also requires
> > kvm_gmem_get_pfn() to acquire filemap invalidation lock [5].
> >
> > kvm_gmem_get_pfn
> > filemap_invalidate_lock_shared(file_inode(file)->i_mapping);
> >
> > However, since kvm_gmem_get_pfn() is called by kvm_tdp_map_page(), which is
> > in turn invoked within kvm_gmem_populate() in TDX, a deadlock occurs on the
> > filemap invalidation lock.
>
> Bringing the prior discussion over to here: it seems wrong that
> kvm_gmem_get_pfn() is getting called within the kvm_gmem_populate()
> chain, because:
>
> 1) kvm_gmem_populate() is specifically passing the gmem PFN down to
> tdx_gmem_post_populate(), but we are throwing it away to grab it
> again kvm_gmem_get_pfn(), which is then creating these locking issues
> that we are trying to work around. If we could simply pass that PFN down
> to kvm_tdp_map_page() (or some variant), then we would not trigger any
> deadlocks in the first place.
Yes, doing kvm_mmu_faultin_pfn() in tdx_gmem_post_populate() is a major flaw.
> 2) kvm_gmem_populate() is intended for pre-boot population of guest
> memory, and allows the post_populate callback to handle setting
> up the architecture-specific preparation, whereas kvm_gmem_get_pfn()
> calls kvm_arch_gmem_prepare(), which is intended to handle post-boot
> setup of private memory. Having kvm_gmem_get_pfn() called as part of
> kvm_gmem_populate() chain brings things 2 things in conflict with
> each other, and TDX seems to be relying on that fact that it doesn't
> implement a handler for kvm_arch_gmem_prepare().
>
> I don't think this hurts anything in the current code, and I don't
> personally see any issue with open-coding the population path if it doesn't
> fit TDX very well, but there was some effort put into making
> kvm_gmem_populate() usable for both TDX/SNP, and if the real issue isn't the
> design of the interface itself, but instead just some inflexibility on the
> KVM MMU mapping side, then it seems more robust to address the latter if
> possible.
>
> Would something like the below be reasonable?
No, polluting the page fault paths is a non-starter for me. TDX really shouldn't
be synthesizing a page fault when it has the PFN in hand. And some of the behavior
that's desirable for pre-faults looks flat out wrong for TDX. E.g. returning '0'
on RET_PF_WRITE_PROTECTED and RET_PF_SPURIOUS (though maybe spurious is fine?).
I would much rather special case this path, because it absolutely is a special
snowflake. This even eliminates several exports of low level helpers that frankly
have no business being used by TDX, e.g. kvm_mmu_reload().
---
arch/x86/kvm/mmu.h | 2 +-
arch/x86/kvm/mmu/mmu.c | 78 ++++++++++++++++++++++++++++++++++++--
arch/x86/kvm/mmu/tdp_mmu.c | 1 -
arch/x86/kvm/vmx/tdx.c | 24 ++----------
4 files changed, 78 insertions(+), 27 deletions(-)
diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
index b4b6860ab971..9cd7a34333af 100644
--- a/arch/x86/kvm/mmu.h
+++ b/arch/x86/kvm/mmu.h
@@ -258,7 +258,7 @@ extern bool tdp_mmu_enabled;
#endif
bool kvm_tdp_mmu_gpa_is_mapped(struct kvm_vcpu *vcpu, u64 gpa);
-int kvm_tdp_map_page(struct kvm_vcpu *vcpu, gpa_t gpa, u64 error_code, u8 *level);
+int kvm_tdp_mmu_map_private_pfn(struct kvm_vcpu *vcpu, gfn_t gfn, kvm_pfn_t pfn);
static inline bool kvm_memslots_have_rmaps(struct kvm *kvm)
{
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 6e838cb6c9e1..bc937f8ed5a0 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -4900,7 +4900,8 @@ int kvm_tdp_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
return direct_page_fault(vcpu, fault);
}
-int kvm_tdp_map_page(struct kvm_vcpu *vcpu, gpa_t gpa, u64 error_code, u8 *level)
+static int kvm_tdp_prefault_page(struct kvm_vcpu *vcpu, gpa_t gpa,
+ u64 error_code, u8 *level)
{
int r;
@@ -4942,7 +4943,6 @@ int kvm_tdp_map_page(struct kvm_vcpu *vcpu, gpa_t gpa, u64 error_code, u8 *level
return -EIO;
}
}
-EXPORT_SYMBOL_GPL(kvm_tdp_map_page);
long kvm_arch_vcpu_pre_fault_memory(struct kvm_vcpu *vcpu,
struct kvm_pre_fault_memory *range)
@@ -4978,7 +4978,7 @@ long kvm_arch_vcpu_pre_fault_memory(struct kvm_vcpu *vcpu,
* Shadow paging uses GVA for kvm page fault, so restrict to
* two-dimensional paging.
*/
- r = kvm_tdp_map_page(vcpu, range->gpa | direct_bits, error_code, &level);
+ r = kvm_tdp_prefault_page(vcpu, range->gpa | direct_bits, error_code, &level);
if (r < 0)
return r;
@@ -4990,6 +4990,77 @@ long kvm_arch_vcpu_pre_fault_memory(struct kvm_vcpu *vcpu,
return min(range->size, end - range->gpa);
}
+int kvm_tdp_mmu_map_private_pfn(struct kvm_vcpu *vcpu, gfn_t gfn, kvm_pfn_t pfn)
+{
+ struct kvm_page_fault fault = {
+ .addr = gfn_to_gpa(gfn),
+ .error_code = PFERR_GUEST_FINAL_MASK | PFERR_PRIVATE_ACCESS,
+ .prefetch = true,
+ .is_tdp = true,
+ .nx_huge_page_workaround_enabled = is_nx_huge_page_enabled(vcpu->kvm),
+
+ .max_level = KVM_MAX_HUGEPAGE_LEVEL,
+ .req_level = PG_LEVEL_4K,
+ .goal_level = PG_LEVEL_4K,
+ .is_private = true,
+
+ .gfn = gfn,
+ .slot = kvm_vcpu_gfn_to_memslot(vcpu, gfn),
+ .pfn = pfn,
+ .map_writable = true,
+ };
+ struct kvm *kvm = vcpu->kvm;
+ int r;
+
+ lockdep_assert_held(&kvm->slots_lock);
+
+ if (KVM_BUG_ON(!tdp_mmu_enabled, kvm))
+ return -EIO;
+
+ if (kvm_gfn_is_write_tracked(kvm, fault.slot, fault.gfn))
+ return -EPERM;
+
+ r = kvm_mmu_reload(vcpu);
+ if (r)
+ return r;
+
+ r = mmu_topup_memory_caches(vcpu, false);
+ if (r)
+ return r;
+
+ do {
+ if (signal_pending(current))
+ return -EINTR;
+
+ if (kvm_test_request(KVM_REQ_VM_DEAD, vcpu))
+ return -EIO;
+
+ cond_resched();
+
+ guard(read_lock)(&kvm->mmu_lock);
+
+ r = kvm_tdp_mmu_map(vcpu, &fault);
+ } while (r == RET_PF_RETRY);
+
+ if (r != RET_PF_FIXED)
+ return -EIO;
+
+ /*
+ * The caller is responsible for ensuring that no MMU invalidations can
+ * occur. Sanity check that the mapping hasn't been zapped.
+ */
+ if (IS_ENABLED(CONFIG_KVM_PROVE_MMU)) {
+ cond_resched();
+
+ scoped_guard(read_lock, &kvm->mmu_lock) {
+ if (KVM_BUG_ON(!kvm_tdp_mmu_gpa_is_mapped(vcpu, fault.addr), kvm))
+ return -EIO;
+ }
+ }
+ return 0;
+}
+EXPORT_SYMBOL_GPL(kvm_tdp_mmu_map_private_pfn);
+
static void nonpaging_init_context(struct kvm_mmu *context)
{
context->page_fault = nonpaging_page_fault;
@@ -5973,7 +6044,6 @@ int kvm_mmu_load(struct kvm_vcpu *vcpu)
out:
return r;
}
-EXPORT_SYMBOL_GPL(kvm_mmu_load);
void kvm_mmu_unload(struct kvm_vcpu *vcpu)
{
diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index 7f3d7229b2c1..4f73d5341ebe 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -1953,7 +1953,6 @@ bool kvm_tdp_mmu_gpa_is_mapped(struct kvm_vcpu *vcpu, u64 gpa)
spte = sptes[leaf];
return is_shadow_present_pte(spte) && is_last_spte(spte, leaf);
}
-EXPORT_SYMBOL_GPL(kvm_tdp_mmu_gpa_is_mapped);
/*
* Returns the last level spte pointer of the shadow page walk for the given
diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
index f4d4fd5cc6e8..02142496754f 100644
--- a/arch/x86/kvm/vmx/tdx.c
+++ b/arch/x86/kvm/vmx/tdx.c
@@ -3170,15 +3170,12 @@ struct tdx_gmem_post_populate_arg {
static int tdx_gmem_post_populate(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn,
void __user *src, int order, void *_arg)
{
- u64 error_code = PFERR_GUEST_FINAL_MASK | PFERR_PRIVATE_ACCESS;
- struct kvm_tdx *kvm_tdx = to_kvm_tdx(kvm);
struct tdx_gmem_post_populate_arg *arg = _arg;
- struct kvm_vcpu *vcpu = arg->vcpu;
+ struct kvm_tdx *kvm_tdx = to_kvm_tdx(kvm);
+ u64 err, entry, level_state;
gpa_t gpa = gfn_to_gpa(gfn);
- u8 level = PG_LEVEL_4K;
struct page *src_page;
int ret, i;
- u64 err, entry, level_state;
/*
* Get the source page if it has been faulted in. Return failure if the
@@ -3190,24 +3187,10 @@ static int tdx_gmem_post_populate(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn,
if (ret != 1)
return -ENOMEM;
- ret = kvm_tdp_map_page(vcpu, gpa, error_code, &level);
+ ret = kvm_tdp_mmu_map_private_pfn(arg->vcpu, gfn, pfn);
if (ret < 0)
goto out;
- /*
- * The private mem cannot be zapped after kvm_tdp_map_page()
- * because all paths are covered by slots_lock and the
- * filemap invalidate lock. Check that they are indeed enough.
- */
- if (IS_ENABLED(CONFIG_KVM_PROVE_MMU)) {
- scoped_guard(read_lock, &kvm->mmu_lock) {
- if (KVM_BUG_ON(!kvm_tdp_mmu_gpa_is_mapped(vcpu, gpa), kvm)) {
- ret = -EIO;
- goto out;
- }
- }
- }
-
ret = 0;
err = tdh_mem_page_add(&kvm_tdx->td, gpa, pfn_to_page(pfn),
src_page, &entry, &level_state);
@@ -3267,7 +3250,6 @@ static int tdx_vcpu_init_mem_region(struct kvm_vcpu *vcpu, struct kvm_tdx_cmd *c
!vt_is_tdx_private_gpa(kvm, region.gpa + (region.nr_pages << PAGE_SHIFT) - 1))
return -EINVAL;
- kvm_mmu_reload(vcpu);
ret = 0;
while (region.nr_pages) {
if (signal_pending(current)) {
base-commit: 6c7ecd725e503bf2ca69ff52c6cc48bb650b1f11
--
