[xen master] x86/pdx: simplify calculation of domain struct allocation boundary

[patchbot@xxxxxxx]

commit a2602f3a6398758d697437b93f944f90304dc1f2
Author:     Roger Pau Monne <roger.pau@xxxxxxxxxx>
AuthorDate: Mon May 26 19:32:21 2025 +0200
Commit:     Roger Pau Monne <roger.pau@xxxxxxxxxx>
CommitDate: Thu Jun 26 10:37:13 2025 +0200

    x86/pdx: simplify calculation of domain struct allocation boundary
    
    When not using CONFIG_BIGMEM there are some restrictions in the address
    width for allocations of the domain structure, as it's PDX truncated to 32
    bits it's stashed into page_info structure for domain allocated pages.
    
    The current logic to calculate this limit is based on the internals of the
    PDX compression used, which is not strictly required.  Instead simplify the
    logic to rely on the existing PDX to PFN conversion helpers used elsewhere.
    
    This has the added benefit of allowing alternative PDX compression
    algorithms to be implemented without requiring to change the calculation of
    the domain structure allocation boundary.
    
    As a side effect introduce pdx_to_paddr() conversion macro and use it.
    
    Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 xen/arch/x86/domain.c | 40 +++++++++++-----------------------------
 xen/include/xen/pdx.h |  1 +
 2 files changed, 12 insertions(+), 29 deletions(-)

diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c
index d025befe3d..b67342797f 100644
--- a/xen/arch/x86/domain.c
+++ b/xen/arch/x86/domain.c
@@ -461,30 +461,6 @@ void domain_cpu_policy_changed(struct domain *d)
     }
 }
 
-#if !defined(CONFIG_BIGMEM) && defined(CONFIG_PDX_COMPRESSION)
-/*
- * The hole may be at or above the 44-bit boundary, so we need to determine
- * the total bit count until reaching 32 significant (not squashed out) bits
- * in PFN representations.
- * Note that the way "bits" gets initialized/updated/bounds-checked guarantees
- * that the function will never return zero, and hence will never be called
- * more than once (which is important due to it being deliberately placed in
- * .init.text).
- */
-static unsigned int __init noinline _domain_struct_bits(void)
-{
-    unsigned int bits = 32 + PAGE_SHIFT;
-    unsigned int sig = hweight32(~pfn_hole_mask);
-    unsigned int mask = pfn_hole_mask >> 32;
-
-    for ( ; bits < BITS_PER_LONG && sig < 32; ++bits, mask >>= 1 )
-        if ( !(mask & 1) )
-            ++sig;
-
-    return bits;
-}
-#endif
-
 struct domain *alloc_domain_struct(void)
 {
     struct domain *d;
@@ -498,14 +474,20 @@ struct domain *alloc_domain_struct(void)
      * On systems with CONFIG_BIGMEM there's no packing, and so there's no
      * such restriction.
      */
-#if defined(CONFIG_BIGMEM) || !defined(CONFIG_PDX_COMPRESSION)
-    const unsigned int bits = IS_ENABLED(CONFIG_BIGMEM) ? 0 :
-                                                          32 + PAGE_SHIFT;
+#if defined(CONFIG_BIGMEM)
+    const unsigned int bits = 0;
 #else
-    static unsigned int __read_mostly bits;
+    static unsigned int __ro_after_init bits;
 
     if ( unlikely(!bits) )
-         bits = _domain_struct_bits();
+         /*
+          * Get the width for the next pfn, and unconditionally subtract one
+          * from it to ensure the used width will not allocate past the PDX
+          * field limit.
+          */
+         bits = flsl(pdx_to_paddr(1UL <<
+                                  (sizeof(frame_table->v.inuse._domain) * 8)))
+                - 1;
 #endif
 
     BUILD_BUG_ON(sizeof(*d) > PAGE_SIZE);
diff --git a/xen/include/xen/pdx.h b/xen/include/xen/pdx.h
index 9faeea3ac9..c1423d64a9 100644
--- a/xen/include/xen/pdx.h
+++ b/xen/include/xen/pdx.h
@@ -99,6 +99,7 @@ bool __mfn_valid(unsigned long mfn);
 #define pdx_to_mfn(pdx) _mfn(pdx_to_pfn(pdx))
 
 #define paddr_to_pdx(pa) pfn_to_pdx(paddr_to_pfn(pa))
+#define pdx_to_paddr(px) pfn_to_paddr(pdx_to_pfn(px))
 
 #ifdef CONFIG_PDX_COMPRESSION
 
--
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