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[Xen-devel] [PATCH v6 08/15] x86/efi: create new early memory allocator

There is a problem with place_string() which is used as early memory
allocator. It gets memory chunks starting from start symbol and goes
down. Sadly this does not work when Xen is loaded using multiboot2
protocol because then the start lives on 1 MiB address and we should
not allocate a memory from below of it. So, I tried to use mem_lower
address calculated by GRUB2. However, this solution works only on some
machines. There are machines in the wild (e.g. Dell PowerEdge R820)
which uses first ~640 KiB for boot services code or data... :-(((
Hence, we need new memory allocator for Xen EFI boot code which is
quite simple and generic and could be used by place_string() and
efi_arch_allocate_mmap_buffer(). I think about following solutions:

1) We could use native EFI allocation functions (e.g. AllocatePool()
   or AllocatePages()) to get memory chunk. However, later (somewhere
   in __start_xen()) we must copy its contents to safe place or reserve
   it in e820 memory map and map it in Xen virtual address space. This
   means that the code referring to Xen command line, loaded modules and
   EFI memory map, mostly in __start_xen(), will be further complicated
   and diverge from legacy BIOS cases. Additionally, both former things
   have to be placed below 4 GiB because their addresses are stored in
   multiboot_info_t structure which has 32-bit relevant members.

2) We may allocate memory area statically somewhere in Xen code which
   could be used as memory pool for early dynamic allocations. Looks
   quite simple. Additionally, it would not depend on EFI at all and
   could be used on legacy BIOS platforms if we need it. However, we
   must carefully choose size of this pool. We do not want increase Xen
   binary size too much and waste too much memory but also we must fit
   at least memory map on x86 EFI platforms. As I saw on small machine,
   e.g. IBM System x3550 M2 with 8 GiB RAM, memory map may contain more
   than 200 entries. Every entry on x86-64 platform is 40 bytes in size.
   So, it means that we need more than 8 KiB for EFI memory map only.
   Additionally, if we use this memory pool for Xen and modules command
   line storage (it would be used when xen.efi is executed as EFI application)
   then we should add, I think, about 1 KiB. In this case, to be on safe
   side, we should assume at least 64 KiB pool for early memory allocations.
   Which is about 4 times of our earlier calculations. However, during
   discussion on Xen-devel Jan Beulich suggested that just in case we should
   use 1 MiB memory pool like it is in original place_string() implementation.
   So, let's use 1 MiB as it was proposed. If we think that we should not
   waste unallocated memory in the pool on running system then we can mark
   this region as __initdata and move all required data to dynamically
   allocated places somewhere in __start_xen().

2a) We could put memory pool into .bss.page_aligned section. Then allocate
    memory chunks starting from the lowest address. After init phase we can
    free unused portion of the memory pool as in case of .init.text or 
    sections. This way we do not need to allocate any space in image file and
    freeing of unused area in the memory pool is very simple.

Now #2a solution is implemented because it is quite simple and requires
limited number of changes, especially in __start_xen().

Signed-off-by: Daniel Kiper <daniel.kiper@xxxxxxxxxx>
v6 - suggestions/fixes:
   - optimize ebmalloc allocator,
   - move ebmalloc machinery to xen/common/efi/boot.c
     (suggested by Jan Beulich),
   - enforce PAGE_SIZE ebmalloc_mem alignment
     (suggested by Jan Beulich),
   - ebmalloc() must allocate properly
     aligned memory regions
     (suggested by Jan Beulich),
   - printk() should use XENLOG_INFO
     (suggested by Jan Beulich).

v4 - suggestions/fixes:
   - move from #2 solution to #2a solution,
   - improve commit message.
 xen/arch/x86/efi/efi-boot.h |   11 +++--------
 xen/arch/x86/efi/stub.c     |    2 ++
 xen/arch/x86/setup.c        |    5 +++--
 xen/common/efi/boot.c       |   35 ++++++++++++++++++++++++++++++++++-
 xen/include/xen/efi.h       |    1 +
 5 files changed, 43 insertions(+), 11 deletions(-)

diff --git a/xen/arch/x86/efi/efi-boot.h b/xen/arch/x86/efi/efi-boot.h
index 10985721..42cc5f8 100644
--- a/xen/arch/x86/efi/efi-boot.h
+++ b/xen/arch/x86/efi/efi-boot.h
@@ -117,7 +117,7 @@ static void __init relocate_trampoline(unsigned long phys)
 static void __init place_string(u32 *addr, const char *s)
-    static char *__initdata alloc = start;
+    char *alloc = NULL;
     if ( s && *s )
@@ -125,7 +125,7 @@ static void __init place_string(u32 *addr, const char *s)
         const char *old = (char *)(long)*addr;
         size_t len2 = *addr ? strlen(old) + 1 : 0;
-        alloc -= len1 + len2;
+        alloc = ebmalloc(len1 + len2);
          * Insert new string before already existing one. This is needed
          * for options passed on the command line to override options from
@@ -208,12 +208,7 @@ static void __init 
efi_arch_process_memory_map(EFI_SYSTEM_TABLE *SystemTable,
 static void *__init efi_arch_allocate_mmap_buffer(UINTN map_size)
-    place_string(&mbi.mem_upper, NULL);
-    mbi.mem_upper -= map_size;
-    mbi.mem_upper &= -__alignof__(EFI_MEMORY_DESCRIPTOR);
-    if ( mbi.mem_upper < xen_phys_start )
-        return NULL;
-    return (void *)(long)mbi.mem_upper;
+    return ebmalloc(map_size);
 static void __init efi_arch_pre_exit_boot(void)
diff --git a/xen/arch/x86/efi/stub.c b/xen/arch/x86/efi/stub.c
index 0bfaa74..014739a 100644
--- a/xen/arch/x86/efi/stub.c
+++ b/xen/arch/x86/efi/stub.c
@@ -9,6 +9,8 @@ bool efi_enabled(unsigned int feature)
     return false;
+void __init free_ebmalloc_unused_mem(void) { }
 void __init efi_init_memory(void) { }
 void efi_update_l4_pgtable(unsigned int l4idx, l4_pgentry_t l4e) { }
diff --git a/xen/arch/x86/setup.c b/xen/arch/x86/setup.c
index 06f3970..5b17783 100644
--- a/xen/arch/x86/setup.c
+++ b/xen/arch/x86/setup.c
@@ -520,6 +520,8 @@ static void noinline init_done(void)
     system_state = SYS_STATE_active;
+    free_ebmalloc_unused_mem();
     /* MUST be done prior to removing .init data. */
@@ -1080,8 +1082,7 @@ void __init noreturn __start_xen(unsigned long mbi_p)
     if ( !xen_phys_start )
         panic("Not enough memory to relocate Xen.");
-    reserve_e820_ram(&boot_e820, efi_enabled(EFI_LOADER) ? mbi->mem_upper : 
-                     __pa(&_end));
+    reserve_e820_ram(&boot_e820, __pa(&_start), __pa(&_end));
     /* Late kexec reservation (dynamic start address). */
diff --git a/xen/common/efi/boot.c b/xen/common/efi/boot.c
index 1ef5d0b..192c0b8 100644
--- a/xen/common/efi/boot.c
+++ b/xen/common/efi/boot.c
@@ -79,6 +79,8 @@ static size_t wstrlen(const CHAR16 * s);
 static int set_color(u32 mask, int bpp, u8 *pos, u8 *sz);
 static bool_t match_guid(const EFI_GUID *guid1, const EFI_GUID *guid2);
+static void *ebmalloc(size_t size);
 static const EFI_BOOT_SERVICES *__initdata efi_bs;
 static UINT32 __initdata efi_bs_revision;
 static EFI_HANDLE __initdata efi_ih;
@@ -1158,7 +1160,38 @@ efi_start(EFI_HANDLE ImageHandle, EFI_SYSTEM_TABLE 
     for( ; ; ); /* not reached */
-#ifndef CONFIG_ARM /* TODO - runtime service support */
+#ifndef CONFIG_ARM
+#define EBMALLOC_SIZE  MB(1)
+static char __section(".bss.page_aligned") __aligned(PAGE_SIZE) 
+static unsigned long __initdata ebmalloc_allocated;
+/* EFI boot allocator. */
+static void __init *ebmalloc(size_t size)
+    void *ptr = ebmalloc_mem + ebmalloc_allocated;
+    ebmalloc_allocated += (size + sizeof(void *) - 1) & 
~((typeof(size))sizeof(void *) - 1);
+    if ( ebmalloc_allocated > sizeof(ebmalloc_mem) )
+        blexit(L"Out of static memory\r\n");
+    return ptr;
+void __init free_ebmalloc_unused_mem(void)
+    unsigned long start, end;
+    start = (unsigned long)ebmalloc_mem + PAGE_ALIGN(ebmalloc_allocated);
+    end = (unsigned long)ebmalloc_mem + sizeof(ebmalloc_mem);
+    destroy_xen_mappings(start, end);
+    init_xenheap_pages(__pa(start), __pa(end));
+    printk(XENLOG_INFO "Freed %lukB unused BSS memory\n", (end - start) >> 10);
 static bool_t __initdata efi_map_uc;
diff --git a/xen/include/xen/efi.h b/xen/include/xen/efi.h
index 68c68a8..6c7f601 100644
--- a/xen/include/xen/efi.h
+++ b/xen/include/xen/efi.h
@@ -32,6 +32,7 @@ struct xenpf_efi_runtime_call;
 struct compat_pf_efi_runtime_call;
 bool efi_enabled(unsigned int feature);
+void free_ebmalloc_unused_mem(void);
 void efi_init_memory(void);
 bool_t efi_rs_using_pgtables(void);
 unsigned long efi_get_time(void);

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