On 7/27/24 01:51, Sean Christopherson wrote:
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.
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
arch/x86/kvm/mmu/mmu.c | 29 +++++++++++++++++++++++++++--
arch/x86/kvm/mmu/paging_tmpl.h | 5 +++++
arch/x86/kvm/mmu/spte.c | 11 -----------
3 files changed, 32 insertions(+), 13 deletions(-)
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 1cdd67707461..7e7b855ce1e1 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -2918,7 +2918,16 @@ 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, without
"with a fast page fault"
Paolo
+ * 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;
@@ -4314,7 +4323,23 @@ 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 b6897916c76b..2e2d87a925ac 100644
--- a/arch/x86/kvm/mmu/paging_tmpl.h
+++ b/arch/x86/kvm/mmu/paging_tmpl.h
@@ -953,6 +953,11 @@ static int FNAME(sync_spte)(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp, int
spte_to_pfn(spte), spte, true, false,
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 9b8795bd2f04..2c5650390d3b 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;
}
