[PATCH v13 41/85] KVM: x86/mmu: Mark pages/folios dirty at the origin of make_spte()

Sean Christopherson
2024-10-10T23:31:50Z

Move the marking of folios dirty from make_spte() out to its callers,
which have access to the _struct page_, not just the underlying pfn.
Once all architectures follow suit, this will allow removing KVM's ugly
hack where KVM elevates the refcount of VM_MIXEDMAP pfns that happen to
be struct page memory.

Tested-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/kvm/mmu/mmu.c | 30 ++++++++++++++++++++++++++++--
arch/x86/kvm/mmu/paging_tmpl.h | 5 +++++
arch/x86/kvm/mmu/spte.c | 11 -----------
3 files changed, 33 insertions(+), 13 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 31a6ae41a6f4..f730870887dd 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -2964,7 +2964,17 @@ static bool kvm_mmu_prefetch_sptes(struct kvm_vcpu *vcpu, gfn_t gfn, u64 *sptep,
for (i = 0; i < nr_pages; i++, gfn++, sptep++) {
mmu_set_spte(vcpu, slot, sptep, access, gfn,
page_to_pfn(pages[i]), NULL);
- kvm_release_page_clean(pages[i]);
+
+ /*
+ * KVM always prefetches writable pages from the primary MMU,
+ * and KVM can make its SPTE writable in the fast page handler,
+ * without notifying the primary MMU. Mark pages/folios dirty
+ * now to ensure file data is written back if it ends up being
+ * written by the guest. Because KVM's prefetching GUPs
+ * writable PTEs, the probability of unnecessary writeback is
+ * extremely low.
+ */
+ kvm_release_page_dirty(pages[i]);
}

return true;
@@ -4360,7 +4370,23 @@ static u8 kvm_max_private_mapping_level(struct kvm *kvm, kvm_pfn_t pfn,
static void kvm_mmu_finish_page_fault(struct kvm_vcpu *vcpu,
struct kvm_page_fault *fault, int r)
{
- kvm_release_pfn_clean(fault->pfn);
+ lockdep_assert_once(lockdep_is_held(&vcpu->kvm->mmu_lock) ||
+ r == RET_PF_RETRY);
+
+ /*
+ * If the page that KVM got from the *primary MMU* is writable, and KVM
+ * installed or reused a SPTE, mark the page/folio dirty. Note, this
+ * may mark a folio dirty even if KVM created a read-only SPTE, e.g. if
+ * the GFN is write-protected. Folios can't be safely marked dirty
+ * outside of mmu_lock as doing so could race with writeback on the
+ * folio. As a result, KVM can't mark folios dirty in the fast page
+ * fault handler, and so KVM must (somewhat) speculatively mark the
+ * folio dirty if KVM could locklessly make the SPTE writable.
+ */
+ if (!fault->map_writable || r == RET_PF_RETRY)
+ kvm_release_pfn_clean(fault->pfn);
+ else
+ kvm_release_pfn_dirty(fault->pfn);
}

static int kvm_mmu_faultin_pfn_private(struct kvm_vcpu *vcpu,
diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h
index 35d0c3f1a789..f4711674c47b 100644
--- a/arch/x86/kvm/mmu/paging_tmpl.h
+++ b/arch/x86/kvm/mmu/paging_tmpl.h
@@ -954,6 +954,11 @@ static int FNAME(sync_spte)(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp, int
spte_to_pfn(spte), spte, true, true,
host_writable, &spte);

+ /*
+ * There is no need to mark the pfn dirty, as the new protections must
+ * be a subset of the old protections, i.e. synchronizing a SPTE cannot
+ * change the SPTE from read-only to writable.
+ */
return mmu_spte_update(sptep, spte);
}

diff --git a/arch/x86/kvm/mmu/spte.c b/arch/x86/kvm/mmu/spte.c
index 8e8d6ee79c8b..f1a50a78badb 100644
--- a/arch/x86/kvm/mmu/spte.c
+++ b/arch/x86/kvm/mmu/spte.c
@@ -277,17 +277,6 @@ bool make_spte(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
mark_page_dirty_in_slot(vcpu->kvm, slot, gfn);
}

- /*
- * If the page that KVM got from the primary MMU is writable, i.e. if
- * it's host-writable, mark the page/folio dirty. As alluded to above,
- * folios can't be safely marked dirty in the fast page fault handler,
- * and so KVM must (somewhat) speculatively mark the folio dirty even
- * though it isn't guaranteed to be written as KVM won't mark the folio
- * dirty if/when the SPTE is made writable.
- */
- if (host_writable)
- kvm_set_pfn_dirty(pfn);
-
*new_spte = spte;
return wrprot;