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Re: [Xen-devel] [PATCH v7 2/9] mm: Place unscrubbed pages at the end of pagelist

Hi Jan,

On 15/08/17 15:51, Jan Beulich wrote:
On 15.08.17 at 16:41, <boris.ostrovsky@xxxxxxxxxx> wrote:
On 08/15/2017 04:18 AM, Jan Beulich wrote:
On 14.08.17 at 16:29, <boris.ostrovsky@xxxxxxxxxx> wrote:
On 08/14/2017 06:37 AM, Jan Beulich wrote:
On 08.08.17 at 23:45, <boris.ostrovsky@xxxxxxxxxx> wrote:
--- a/xen/include/asm-x86/mm.h
+++ b/xen/include/asm-x86/mm.h
@@ -88,7 +88,15 @@ struct page_info
         /* Page is on a free list: ((count_info & PGC_count_mask) == 0). */
         struct {
             /* Do TLBs need flushing for safety before next page use? */
-            bool_t need_tlbflush;
+            bool need_tlbflush:1;
+            /*
+             * Index of the first *possibly* unscrubbed page in the buddy.
+             * One more bit than maximum possible order to accommodate
+             * INVALID_DIRTY_IDX.
+             */
+#define INVALID_DIRTY_IDX ((1UL << (MAX_ORDER + 1)) - 1)
+            unsigned long first_dirty:MAX_ORDER + 1;
         } free;
I think generated code will be better with the two fields swapped:
That way reading first_dirty won't involve a shift, and accessing a
single bit doesn't require shifts at all on many architectures.
Ok, I will then keep need_tlbflush as the last field so the final struct
(as defined in patch 7) will look like

struct {
        unsigned long first_dirty:MAX_ORDER + 1;
        unsigned long scrub_state:2;
        bool need_tlbflush:1;
Hmm, actually - why do you need bitfields on the x86 side at all?
They're needed for 32-bit architectures only, 64-bit ones ought
to be fine with

struct {
        unsigned int first_dirty;
        bool need_tlbflush;
        uint8_t scrub_state;

IIRC it was exactly because of ARM32 and at some point you suggested to
switch both x86 and ARM to bitfields.

I don't recall for sure whether I had asked for the change to be done
uniformly; it was certainly ARM32 that triggered me notice the
structure size change in your original version.

(plus a suitable BUILD_BUG_ON() to make sure first_dirty has
at least MAX_ORDER + 1 bits). The ARM maintainers will know
whether they would want to also differentiate ARM32 and
ARM64 here.

Isn't using bitfields the only possibility for 32-bit? We can't fit
first_dirty into 2 bytes.

Yes, hence the question whether to stay with bitfields uniformly
or make ARM64 follow x86, but ARM32 keep using bitfields.

I would prefer to avoid differentiation between Arm32 and Arm64.


Julien Grall

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