[PATCH 1/3] x86: introduce "brk" allocator

[Jan Beulich <jbeulich@xxxxxxxx>]

... to replace ebmalloc(), and then to find further use(s) to allow
recovering memory which is needed very early (and hence needs setting up
statically), but may not fully be used (or not used at all).

Note that unlike free_ebmalloc_unused_mem(), brk_free_unused() (once
other code is converted) will be able to free part of the BRK space even
in the xen.efi case. That would happen if BRK space extends across a 2Mb
boundary, and actual use stops before that boundary.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
Changing setup.c's reserve_e820_ram() uses would be cumbersome when done
right here. That'll be done when ebmalloc() is replaced, and hence
what's there can also simply be replaced.

The xen.efi detection may want separating out into a helper.

When linking xen.efi, ld produces a base relocation for the reference to
__subsystem__, which is wrong (that's an absolute symbol, after all).
While that will need fixing there, it does no harm for our purposes.

--- a/xen/arch/x86/boot/Makefile
+++ b/xen/arch/x86/boot/Makefile
@@ -1,4 +1,5 @@
 obj-bin-y += head.o
+obj-bin-y += brk.init.o
 obj-bin-y += built-in-32.o
 obj-bin-y += $(obj64)
 
--- /dev/null
+++ b/xen/arch/x86/boot/brk.c
@@ -0,0 +1,72 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+
+#include <xen/efi.h>
+#include <xen/lib.h>
+#include <xen/mm.h>
+#include <xen/page-defs.h>
+
+#include <asm/brk.h>
+
+extern char __brk_start[];
+extern const char __bss_end[];
+
+static unsigned long __initdata allocated;
+static bool __initdata finished;
+
+void *__init brk_alloc(size_t size)
+{
+    void *ptr = __brk_start + allocated;
+
+    if ( finished )
+        return NULL;
+
+    /* Allocations PAGE_SIZE and up will be page-aligned. */
+    if ( size >= PAGE_SIZE )
+        allocated = ROUNDUP(allocated, PAGE_SIZE);
+
+    allocated += ROUNDUP(size, sizeof(void *));
+
+    if ( allocated > __bss_end - __brk_start )
+        return NULL;
+
+    return ptr;
+}
+
+unsigned long __init brk_get_unused_start(void)
+{
+    finished = true;
+
+    allocated = ROUNDUP(allocated, PAGE_SIZE);
+
+    return (unsigned long)__brk_start + allocated;
+}
+
+void __init brk_free_unused(void)
+{
+    unsigned long start = brk_get_unused_start(),
+                  end = (unsigned long)__bss_end;
+    unsigned int subsys;
+
+    /*
+     * Only xen.efi will have the symbol __subsystem__ available, and it'll
+     * be non-zero (10) there.  In ELF the symbol will be undefined, and
+     * hence zero will be loaded into the register.
+     */
+    asm ( ".weak __subsystem__; mov $__subsystem__, %0" : "=r" (subsys) );
+
+    /* using_2M_mapping() isn't available here. */
+    if ( IS_ENABLED(CONFIG_XEN_ALIGN_2M) || subsys )
+        start = PAGE_ALIGN_2M(start);
+
+    if ( start >= end )
+        return;
+
+    destroy_xen_mappings(start, PAGE_ALIGN_2M(end));
+
+    /*
+     * By reserving needed space early in the E820 map, excess space gets freed
+     * way before we make it here. Don't free the range a 2nd time.
+     */
+
+    printk(XENLOG_INFO "Freed %lukB unused BRK memory\n", (end - start) >> 10);
+}
--- /dev/null
+++ b/xen/arch/x86/include/asm/brk.h
@@ -0,0 +1,7 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+
+#include <xen/types.h>
+
+void *brk_alloc(size_t size);
+unsigned long brk_get_unused_start(void);
+void brk_free_unused(void);
--- a/xen/arch/x86/xen.lds.S
+++ b/xen/arch/x86/xen.lds.S
@@ -331,7 +331,11 @@ SECTIONS
        __bss_start = .;
        *(.bss.page_aligned*)
        PERCPU_BSS
-       *(.bss .bss.*)
+       *(.bss .bss.[a-zA-Z0-9_]*)
+       . = ALIGN(PAGE_SIZE);
+       __brk_start = .;
+       *(.bss..brk.page_aligned*)
+       *(.bss..brk*)
        . = ALIGN(POINTER_ALIGN);
        __bss_end = .;
   } PHDR(text)
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -112,6 +112,7 @@
 #include <xen/vmap.h>
 #include <xen/xmalloc.h>
 
+#include <asm/brk.h>
 #include <asm/e820.h>
 #include <asm/fixmap.h>
 #include <asm/flushtlb.h>
@@ -337,6 +338,8 @@ void __init arch_init_memory(void)
 
     efi_init_memory();
 
+    brk_free_unused();
+
 #ifndef NDEBUG
     if ( highmem_start )
     {