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Re: [RFC PATCH] xen/gnttab: Store frame GFN in struct page_info on Arm

On 14.09.21 11:31, Jan Beulich wrote:

Hi Jan

On 13.09.2021 19:09, Oleksandr wrote:
On 10.09.21 10:52, Jan Beulich wrote:
On 10.09.2021 01:04, Oleksandr Tyshchenko wrote:
@@ -731,11 +733,19 @@ static void p2m_put_l3_page(const lpae_t pte)
       if ( p2m_is_foreign(pte.p2m.type) )
-        mfn_t mfn = lpae_get_mfn(pte);
+    /*
+     * Check whether we deal with grant table page. As the grant table page
+     * is xen_heap page and its entry has known p2m type, detect it and mark
+     * the stored GFN as invalid.
+     */
+    if ( p2m_is_ram(pte.p2m.type) && is_xen_heap_mfn(mfn) )
+        page_set_frame_gfn(mfn_to_page(mfn), INVALID_GFN);
I take it the write done is benign for other Xen heap pages? I suppose
this would want making very explicit, as such an assumption is easy to
go stale by entirely unrelated changes.
Yes, I don't see what bad could happen if we would clear this field for
non grant table pages. Except grant table pages I could detect only one
page passed this check here which is, I assume, shared_info page. All
pages have this field initialized with INVALID_GFN. But *only* grant
table pages have this field in use. So, I think, no one will suffer. I
will add a comment. Or you meant something more than just a comment?
The answer here goes hand in hand with the pending question of whether
you wouldn't better generally introduce recording of the GFN (and hence
effectively an M2P): The less special casing here, the better imo. The
more special casing here, the more justification / explanation is imo
needed in the comment.

I got your point. I think, I am not the right person to judge whether the Xen on Arm needs / wants complete M2P support or not (this is a question to the maintainers and the community), but I can't recollect a strong desire or critical need for implementing it every time such discussion arose and IIRC only arguments against it. Also this activity is not in my list of priorities to do on Xen on Arm. But, I agree that having M2P would prevent this (with grant table) and similar situations. So, I will add more justification / explanation for the current patch.

--- a/xen/include/asm-arm/mm.h
+++ b/xen/include/asm-arm/mm.h
@@ -39,7 +39,15 @@ struct page_info
           /* Page is in use: ((count_info & PGC_count_mask) != 0). */
           struct {
               /* Type reference count and various PGT_xxx flags and fields. */
-            unsigned long type_info;
+            unsigned long type_info:4;
+            /*
+             * Stored GFN if page is used for grant table frame
+             * (bits 59(27)-0).
Are these bit numbers correct? I thought Arm, like x86, counted bits
from low to high (unlike PPC, for example)?
I think, these are correct.  Yes, Little Endian is used.
Well, the low 4 bits are used by type_info here. Therefore I'm having
trouble seeing what the 0 refers to.

Now I see that comment is not precise. Probably I should have written that "60-bit(28-bit) field is used to store the GFN ..."

@@ -94,12 +102,12 @@ struct page_info
   #define PG_shift(idx)   (BITS_PER_LONG - (idx))
   #define PG_mask(x, idx) (x ## UL << PG_shift(idx))
-#define PGT_none PG_mask(0, 1) /* no special uses of this page */
-#define PGT_writable_page PG_mask(1, 1)  /* has writable mappings?         */
-#define PGT_type_mask     PG_mask(1, 1)  /* Bits 31 or 63.                 */
+#define PGT_none          (0UL << 3)  /* no special uses of this page   */
+#define PGT_writable_page (1UL << 3)  /* has writable mappings?         */
+#define PGT_type_mask     (1UL << 3)
/* Count of uses of this frame as its current type. */
-#define PGT_count_width   PG_shift(2)
+#define PGT_count_width   1
   #define PGT_count_mask    ((1UL<<PGT_count_width)-1)
Leaving just a single bit seems pretty risky to me, and I can't see
why you do so. You need 52 bits on Arm64. Which means you have 12
bits left. Why not use them in full? Even on Arm32 you have 3 bits
available for the count afaict.
Only 1 bit in the type_info field is really used on Arm, but anyway ...

Also there's a disconnect here: If anyone wanted to change the number
of bits used for the various fields, each such change should require
an adjustment in as few independent places as possible. That is, there
shouldn't be multiple uses of literal 4 further up, and there also
shouldn't be a hidden connection between that 4 and the PGT_* values
here. I think you'd better express the GFN as such a PGT_* construct
as well. And you better would keep using PG_mask() and PG_shift().
... I think, this indeed makes sense. If got your request correctly, we
can avoid disconnect here
and reserve 3-bit field for count for both Arm32 and Arm64. The struct
page_info's type_info field remains untouched.

diff --git a/xen/include/asm-arm/mm.h b/xen/include/asm-arm/mm.h
index ded74d2..8b73e1c 100644
--- a/xen/include/asm-arm/mm.h
+++ b/xen/include/asm-arm/mm.h
@@ -99,8 +99,14 @@ struct page_info
   #define PGT_type_mask     PG_mask(1, 1)  /* Bits 31 or
63.                 */

    /* Count of uses of this frame as its current type. */
-#define PGT_count_width   PG_shift(2)
-#define PGT_count_mask    ((1UL<<PGT_count_width)-1)
+#define PGT_count_base    PG_shift(4)
+#define PGT_count_mask    PG_mask(7, 4)
+/* Stored in bits [27:0] or [59:0] GFN if page is used for grant table
frame */
+#define PGT_gfn_width     PG_shift(4)
+#define PGT_gfn_mask      ((1UL<<PGT_gfn_width)-1)
+#define PGT_INVALID_FRAME_GFN   _gfn(PGT_gfn_mask)

    /* Cleared when the owning guest 'frees' this page. */
   #define _PGC_allocated    PG_shift(1)
@@ -163,6 +169,22 @@ extern unsigned long xenheap_base_pdx;

   #define maddr_get_owner(ma) (page_get_owner(maddr_to_page((ma))))

+#define page_get_frame_gfn(_p) ({                               \
+    gfn_t gfn_ = _gfn((_p)->u.inuse.type_info & PGT_gfn_mask);  \
+    gfn_eq(gfn_, PGT_INVALID_FRAME_GFN) ? INVALID_GFN : gfn_;   \
+#define page_set_frame_gfn(_p, _gfn) ({                         \
+    gfn_t gfn_ = gfn_eq(_gfn, INVALID_GFN) ?                    \
+                 PGT_INVALID_FRAME_GFN : _gfn;                  \
+    (_p)->u.inuse.type_info &= ~PGT_gfn_mask;                   \
+    (_p)->u.inuse.type_info |= gfn_x(gfn_);                     \


Is it close to what you keep in mind?
Yes. Just to be sure - did you check that moving the count up
from starting at bit 0 is compatible with all present uses? At
least on x86 I think we have uses where it is assumed to occupy
the bottom so many bits (and the same also for PGC_count_mask).

ok, thank you. Yes, I checked that (on Arm). I could find only one place which would need updating, which was the share_xen_page_with_guest().



Oleksandr Tyshchenko



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