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Thu, 13 Nov 2025 03:08:20 -0800 (PST) Message-ID: <7a3eb7f3-db3e-4c2f-a231-cdf05a14be26@suse.com> Date: Thu, 13 Nov 2025 12:08:18 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: [PATCH 1/3] x86: introduce "brk" allocator From: Jan Beulich To: "xen-devel@lists.xenproject.org" Cc: Andrew Cooper , =?UTF-8?Q?Roger_Pau_Monn=C3=A9?= References: Content-Language: en-US Autocrypt: addr=jbeulich@suse.com; keydata= xsDiBFk3nEQRBADAEaSw6zC/EJkiwGPXbWtPxl2xCdSoeepS07jW8UgcHNurfHvUzogEq5xk hu507c3BarVjyWCJOylMNR98Yd8VqD9UfmX0Hb8/BrA+Hl6/DB/eqGptrf4BSRwcZQM32aZK 7Pj2XbGWIUrZrd70x1eAP9QE3P79Y2oLrsCgbZJfEwCgvz9JjGmQqQkRiTVzlZVCJYcyGGsD /0tbFCzD2h20ahe8rC1gbb3K3qk+LpBtvjBu1RY9drYk0NymiGbJWZgab6t1jM7sk2vuf0Py O9Hf9XBmK0uE9IgMaiCpc32XV9oASz6UJebwkX+zF2jG5I1BfnO9g7KlotcA/v5ClMjgo6Gl MDY4HxoSRu3i1cqqSDtVlt+AOVBJBACrZcnHAUSuCXBPy0jOlBhxPqRWv6ND4c9PH1xjQ3NP nxJuMBS8rnNg22uyfAgmBKNLpLgAGVRMZGaGoJObGf72s6TeIqKJo/LtggAS9qAUiuKVnygo 3wjfkS9A3DRO+SpU7JqWdsveeIQyeyEJ/8PTowmSQLakF+3fote9ybzd880fSmFuIEJldWxp Y2ggPGpiZXVsaWNoQHN1c2UuY29tPsJgBBMRAgAgBQJZN5xEAhsDBgsJCAcDAgQVAggDBBYC AwECHgECF4AACgkQoDSui/t3IH4J+wCfQ5jHdEjCRHj23O/5ttg9r9OIruwAn3103WUITZee e7Sbg12UgcQ5lv7SzsFNBFk3nEQQCACCuTjCjFOUdi5Nm244F+78kLghRcin/awv+IrTcIWF hUpSs1Y91iQQ7KItirz5uwCPlwejSJDQJLIS+QtJHaXDXeV6NI0Uef1hP20+y8qydDiVkv6l IreXjTb7DvksRgJNvCkWtYnlS3mYvQ9NzS9PhyALWbXnH6sIJd2O9lKS1Mrfq+y0IXCP10eS FFGg+Av3IQeFatkJAyju0PPthyTqxSI4lZYuJVPknzgaeuJv/2NccrPvmeDg6Coe7ZIeQ8Yj t0ARxu2xytAkkLCel1Lz1WLmwLstV30g80nkgZf/wr+/BXJW/oIvRlonUkxv+IbBM3dX2OV8 AmRv1ySWPTP7AAMFB/9PQK/VtlNUJvg8GXj9ootzrteGfVZVVT4XBJkfwBcpC/XcPzldjv+3 HYudvpdNK3lLujXeA5fLOH+Z/G9WBc5pFVSMocI71I8bT8lIAzreg0WvkWg5V2WZsUMlnDL9 mpwIGFhlbM3gfDMs7MPMu8YQRFVdUvtSpaAs8OFfGQ0ia3LGZcjA6Ik2+xcqscEJzNH+qh8V m5jjp28yZgaqTaRbg3M/+MTbMpicpZuqF4rnB0AQD12/3BNWDR6bmh+EkYSMcEIpQmBM51qM EKYTQGybRCjpnKHGOxG0rfFY1085mBDZCH5Kx0cl0HVJuQKC+dV2ZY5AqjcKwAxpE75MLFkr wkkEGBECAAkFAlk3nEQCGwwACgkQoDSui/t3IH7nnwCfcJWUDUFKdCsBH/E5d+0ZnMQi+G0A nAuWpQkjM1ASeQwSHEeAWPgskBQL In-Reply-To: Content-Transfer-Encoding: quoted-printable X-ZohoMail-DKIM: pass (identity @suse.com) X-ZM-MESSAGEID: 1763032128256153000 Content-Type: text/plain; charset="utf-8" ... 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 --- 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 +=3D head.o +obj-bin-y +=3D brk.init.o obj-bin-y +=3D built-in-32.o obj-bin-y +=3D $(obj64) =20 --- /dev/null +++ b/xen/arch/x86/boot/brk.c @@ -0,0 +1,72 @@ +/* SPDX-License-Identifier: GPL-2.0-or-later */ + +#include +#include +#include +#include + +#include + +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 =3D __brk_start + allocated; + + if ( finished ) + return NULL; + + /* Allocations PAGE_SIZE and up will be page-aligned. */ + if ( size >=3D PAGE_SIZE ) + allocated =3D ROUNDUP(allocated, PAGE_SIZE); + + allocated +=3D ROUNDUP(size, sizeof(void *)); + + if ( allocated > __bss_end - __brk_start ) + return NULL; + + return ptr; +} + +unsigned long __init brk_get_unused_start(void) +{ + finished =3D true; + + allocated =3D ROUNDUP(allocated, PAGE_SIZE); + + return (unsigned long)__brk_start + allocated; +} + +void __init brk_free_unused(void) +{ + unsigned long start =3D brk_get_unused_start(), + end =3D (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" : "=3Dr" (subsys) = ); + + /* using_2M_mapping() isn't available here. */ + if ( IS_ENABLED(CONFIG_XEN_ALIGN_2M) || subsys ) + start =3D PAGE_ALIGN_2M(start); + + if ( start >=3D 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 + +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 =3D .; *(.bss.page_aligned*) PERCPU_BSS - *(.bss .bss.*) + *(.bss .bss.[a-zA-Z0-9_]*) + . =3D ALIGN(PAGE_SIZE); + __brk_start =3D .; + *(.bss..brk.page_aligned*) + *(.bss..brk*) . =3D ALIGN(POINTER_ALIGN); __bss_end =3D .; } PHDR(text) --- a/xen/arch/x86/mm.c +++ b/xen/arch/x86/mm.c @@ -112,6 +112,7 @@ #include #include =20 +#include #include #include #include @@ -337,6 +338,8 @@ void __init arch_init_memory(void) =20 efi_init_memory(); =20 + brk_free_unused(); + #ifndef NDEBUG if ( highmem_start ) {