[RFC please check] KVM: question about the commit "Use Little Endian for Dirty Bitmap"

[Takuya Yoshikawa <yoshikawa.takuya@xxxxxxxxxxxxx>]

Hi, Alex, Avi, Marcelo

I would like to ask you about the commit:
  c8240bd6f0b4b1b21ffd36dd44114d05c7afe0c0 "Use Little Endian for Dirty Bitmap"
I pasted a snipet below!

I am now confused by the Alex's comment to my recent patch:
  "change mark_page_dirty() to handle endian issues explicitly"
in which I open-coded the generic___set_le_bit().

So please explain me about the commit:
  1. is this really the thing you intended to do?
  2. including <asm-generic/bitops/le.h> directly is OK?
     -- I made a sample patch to avoid this, see below.

  3. or, I misunderstand something about Alex's comment?

Thanks,
  Takuya


=== snipet from commit c8240bd6f0b4b1b21ffd36dd44114d05c7afe0c0 ===
@@ -48,6 +48,7 @@
 #include <asm/io.h>
 #include <asm/uaccess.h>
 #include <asm/pgtable.h>
+#include <asm-generic/bitops/le.h>
 
 #ifdef KVM_COALESCED_MMIO_PAGE_OFFSET
 #include "coalesced_mmio.h"
@@ -1665,8 +1666,8 @@ void mark_page_dirty(struct kvm *kvm, gfn_t gfn)
 		unsigned long rel_gfn = gfn - memslot->base_gfn;
 
 		/* avoid RMW */
-		if (!test_bit(rel_gfn, memslot->dirty_bitmap))
-			set_bit(rel_gfn, memslot->dirty_bitmap);
+		if (!generic_test_le_bit(rel_gfn, memslot->dirty_bitmap))
+			generic___set_le_bit(rel_gfn, memslot->dirty_bitmap);
 	}
 }

===


=== not tested ===
[PATCH sample] KVM: avoid to include an asm-generic bitops header file directly

Including asm-generic bitops headers is kind of violation: there is no guarantee that
no one will change those functions we are using.

Signed-off-by: Takuya Yoshikawa <yoshikawatky@xxxxxxxxxxxxxxxxxxxxxxxxxx>
---
 virt/kvm/kvm_main.c |   17 +++++++++++++----
 1 files changed, 13 insertions(+), 4 deletions(-)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 3725605..f029760 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -51,7 +51,6 @@
 #include <asm/io.h>
 #include <asm/uaccess.h>
 #include <asm/pgtable.h>
-#include <asm-generic/bitops/le.h>
 
 #include "coalesced_mmio.h"
 
@@ -1179,6 +1178,18 @@ int kvm_clear_guest(struct kvm *kvm, gpa_t gpa, unsigned long len)
 }
 EXPORT_SYMBOL_GPL(kvm_clear_guest);
 
+/*
+ * __set_le_bit is defined for ppc only.
+ */
+static void kvm___set_le_bit(unsigned long nr, unsigned long *addr)
+{
+#ifdef __set_le_bit
+	__set_le_bit(nr, addr);
+#else
+	__set_bit(nr, addr);
+#endif
+}
+
 void mark_page_dirty(struct kvm *kvm, gfn_t gfn)
 {
 	struct kvm_memory_slot *memslot;
@@ -1188,9 +1199,7 @@ void mark_page_dirty(struct kvm *kvm, gfn_t gfn)
 	if (memslot && memslot->dirty_bitmap) {
 		unsigned long rel_gfn = gfn - memslot->base_gfn;
 
-		/* avoid RMW */
-		if (!generic_test_le_bit(rel_gfn, memslot->dirty_bitmap))
-			generic___set_le_bit(rel_gfn, memslot->dirty_bitmap);
+		kvm___set_le_bit(rel_gfn, memslot->dirty_bitmap);
 	}
 }
 
-- 
1.6.3.3

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