[Xen-devel] [PATCH] xen/x86: replace order-based range checking of M2P table by linear one

["Jan Beulich" <JBeulich@xxxxxxxxxx>]

The order-based approach is not only less efficient (requiring a shift
and a compare, typical generated code looking like this

        mov     eax, [machine_to_phys_order]
        mov     ecx, eax
        shr     ebx, cl
        test    ebx, ebx
        jnz     ...

whereas a direct check requires just a compare, like in

        cmp     ebx, [machine_to_phys_nr]
        jae     ...

), but also slightly dangerous in the 32-on-64 case - the element
address calculation can wrap if the next power of two boundary is
sufficiently far away from the actual upper limit of the table, and
hence can result in user space addresses being accessed (with it being
unknown what may actually be mapped there).

Additionally, the elimination of the mistaken use of fls() here (should
have been __fls()) fixes a latent issue on x86-64 that would trigger
if the code was run on a system with memory extending beyond the 44-bit
boundary.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxxxx>

---
 arch/x86/include/asm/xen/page.h |    4 ++--
 arch/x86/xen/enlighten.c        |    4 ++--
 arch/x86/xen/mmu.c              |   12 ++++++++----
 3 files changed, 12 insertions(+), 8 deletions(-)

--- 3.1-rc2/arch/x86/include/asm/xen/page.h
+++ 3.1-rc2-x86-xen-p2m-nr/arch/x86/include/asm/xen/page.h
@@ -39,7 +39,7 @@ typedef struct xpaddr {
     ((unsigned long)((u64)CONFIG_XEN_MAX_DOMAIN_MEMORY * 1024 * 1024 * 1024 / 
PAGE_SIZE))
 
 extern unsigned long *machine_to_phys_mapping;
-extern unsigned int   machine_to_phys_order;
+extern unsigned long  machine_to_phys_nr;
 
 extern unsigned long get_phys_to_machine(unsigned long pfn);
 extern bool set_phys_to_machine(unsigned long pfn, unsigned long mfn);
@@ -87,7 +87,7 @@ static inline unsigned long mfn_to_pfn(u
        if (xen_feature(XENFEAT_auto_translated_physmap))
                return mfn;
 
-       if (unlikely((mfn >> machine_to_phys_order) != 0)) {
+       if (unlikely(mfn >= machine_to_phys_nr)) {
                pfn = ~0;
                goto try_override;
        }
--- 3.1-rc2/arch/x86/xen/enlighten.c
+++ 3.1-rc2-x86-xen-p2m-nr/arch/x86/xen/enlighten.c
@@ -77,8 +77,8 @@ EXPORT_SYMBOL_GPL(xen_domain_type);
 
 unsigned long *machine_to_phys_mapping = (void *)MACH2PHYS_VIRT_START;
 EXPORT_SYMBOL(machine_to_phys_mapping);
-unsigned int   machine_to_phys_order;
-EXPORT_SYMBOL(machine_to_phys_order);
+unsigned long  machine_to_phys_nr;
+EXPORT_SYMBOL(machine_to_phys_nr);
 
 struct start_info *xen_start_info;
 EXPORT_SYMBOL_GPL(xen_start_info);
--- 3.1-rc2/arch/x86/xen/mmu.c
+++ 3.1-rc2-x86-xen-p2m-nr/arch/x86/xen/mmu.c
@@ -1713,15 +1713,19 @@ static void __init xen_map_identity_earl
 void __init xen_setup_machphys_mapping(void)
 {
        struct xen_machphys_mapping mapping;
-       unsigned long machine_to_phys_nr_ents;
 
        if (HYPERVISOR_memory_op(XENMEM_machphys_mapping, &mapping) == 0) {
                machine_to_phys_mapping = (unsigned long *)mapping.v_start;
-               machine_to_phys_nr_ents = mapping.max_mfn + 1;
+               machine_to_phys_nr = mapping.max_mfn + 1;
        } else {
-               machine_to_phys_nr_ents = MACH2PHYS_NR_ENTRIES;
+               machine_to_phys_nr = MACH2PHYS_NR_ENTRIES;
        }
-       machine_to_phys_order = fls(machine_to_phys_nr_ents - 1);
+#ifdef CONFIG_X86_32
+       if (machine_to_phys_mapping + machine_to_phys_nr
+           < machine_to_phys_mapping)
+               machine_to_phys_nr = (unsigned long *)NULL
+                                    - machine_to_phys_mapping;
+#endif
 }
 
 #ifdef CONFIG_X86_64

Attachment: linux-3.1-rc2-x86-xen-p2m-nr.patch
Description: Text document

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