KVM: x86/mmu: Clean up function comments for dirty logging APIs

Rework the function comment for kvm_arch_mmu_enable_log_dirty_pt_masked()
into the body of the function, as it has gotten a bit stale, is harder to
read without the code context, and is the last source of warnings for W=1
builds in KVM x86 due to using a kernel-doc comment without documenting
all parameters.

Opportunistically subsume the functions comments for
kvm_mmu_write_protect_pt_masked() and kvm_mmu_clear_dirty_pt_masked(), as
there is no value in regurgitating similar information at a higher level,
and capturing the differences between write-protection and PML-based dirty
logging is best done in a common location.

No functional change intended.

Cc: David Matlack <dmatlack@google.com>
Reviewed-by: Kai Huang <kai.huang@intel.com>
Reviewed-by: Pankaj Gupta <pankaj.gupta@amd.com>
Link: https://lore.kernel.org/r/20240802202006.340854-1-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 928cf84778b0c5470b64c79132f4ecc9906891e0..5226bb055d99cbc871a60d8a97914a1f2b9be8be 100644 (file)
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -1307,15 +1307,6 @@ static bool __rmap_clear_dirty(struct kvm *kvm, struct kvm_rmap_head *rmap_head,
        return flush;
 }
 
-/**
- * kvm_mmu_write_protect_pt_masked - write protect selected PT level pages
- * @kvm: kvm instance
- * @slot: slot to protect
- * @gfn_offset: start of the BITS_PER_LONG pages we care about
- * @mask: indicates which pages we should protect
- *
- * Used when we do not need to care about huge page mappings.
- */
 static void kvm_mmu_write_protect_pt_masked(struct kvm *kvm,
                                     struct kvm_memory_slot *slot,
                                     gfn_t gfn_offset, unsigned long mask)
@@ -1339,16 +1330,6 @@ static void kvm_mmu_write_protect_pt_masked(struct kvm *kvm,
        }
 }
 
-/**
- * kvm_mmu_clear_dirty_pt_masked - clear MMU D-bit for PT level pages, or write
- * protect the page if the D-bit isn't supported.
- * @kvm: kvm instance
- * @slot: slot to clear D-bit
- * @gfn_offset: start of the BITS_PER_LONG pages we care about
- * @mask: indicates which pages we should clear D-bit
- *
- * Used for PML to re-log the dirty GPAs after userspace querying dirty_bitmap.
- */
 static void kvm_mmu_clear_dirty_pt_masked(struct kvm *kvm,
                                         struct kvm_memory_slot *slot,
                                         gfn_t gfn_offset, unsigned long mask)
@@ -1372,24 +1353,16 @@ static void kvm_mmu_clear_dirty_pt_masked(struct kvm *kvm,
        }
 }
 
-/**
- * kvm_arch_mmu_enable_log_dirty_pt_masked - enable dirty logging for selected
- * PT level pages.
- *
- * It calls kvm_mmu_write_protect_pt_masked to write protect selected pages to
- * enable dirty logging for them.
- *
- * We need to care about huge page mappings: e.g. during dirty logging we may
- * have such mappings.
- */
 void kvm_arch_mmu_enable_log_dirty_pt_masked(struct kvm *kvm,
                                struct kvm_memory_slot *slot,
                                gfn_t gfn_offset, unsigned long mask)
 {
        /*
-        * Huge pages are NOT write protected when we start dirty logging in
-        * initially-all-set mode; must write protect them here so that they
-        * are split to 4K on the first write.
+        * If the slot was assumed to be "initially all dirty", write-protect
+        * huge pages to ensure they are split to 4KiB on the first write (KVM
+        * dirty logs at 4KiB granularity). If eager page splitting is enabled,
+        * immediately try to split huge pages, e.g. so that vCPUs don't get
+        * saddled with the cost of splitting.
         *
         * The gfn_offset is guaranteed to be aligned to 64, but the base_gfn
         * of memslot has no such restriction, so the range can cross two large
@@ -1411,7 +1384,16 @@ void kvm_arch_mmu_enable_log_dirty_pt_masked(struct kvm *kvm,
                                                       PG_LEVEL_2M);
        }
 
-       /* Now handle 4K PTEs.  */
+       /*
+        * (Re)Enable dirty logging for all 4KiB SPTEs that map the GFNs in
+        * mask.  If PML is enabled and the GFN doesn't need to be write-
+        * protected for other reasons, e.g. shadow paging, clear the Dirty bit.
+        * Otherwise clear the Writable bit.
+        *
+        * Note that kvm_mmu_clear_dirty_pt_masked() is called whenever PML is
+        * enabled but it chooses between clearing the Dirty bit and Writeable
+        * bit based on the context.
+        */
        if (kvm_x86_ops.cpu_dirty_log_size)
                kvm_mmu_clear_dirty_pt_masked(kvm, slot, gfn_offset, mask);
        else