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b=LZBtRzriTRgs2q0SMgN0GUCLfYv1dLEfA5c2NvwJxED7//hOC3pz2IYGKdpunV/201n/BbaFkvGtE7qsxZJzZzqrgap6NCocpAeU9K+VHT0JuLlKhWrzxVflmTZbmkcnPml6eSaf9IqXwMqeYaC9MR3kjyI1eBfAms03DWQ9V7A= Authentication-Results: dkim=none (message not signed) header.d=none;dmarc=none action=none header.from=citrix.com; Message-ID: <91bdef16-2da7-49e7-af5a-e395da533330@citrix.com> Date: Thu, 30 Jul 2026 18:59:20 +0100 User-Agent: Mozilla Thunderbird Cc: Andrew Cooper , Teddy Astie , =?UTF-8?Q?Roger_Pau_Monn=C3=A9?= Subject: Re: [PATCH v2 1/3] x86: introduce "brk" allocator To: Jan Beulich , "xen-devel@lists.xenproject.org" References: <31ba7ff2-a42d-44aa-976c-17fbff44a56f@suse.com> Content-Language: en-GB From: Andrew Cooper Autocrypt: addr=andrew.cooper3@citrix.com; keydata= xsFNBFLhNn8BEADVhE+Hb8i0GV6mihnnr/uiQQdPF8kUoFzCOPXkf7jQ5sLYeJa0cQi6Penp VtiFYznTairnVsN5J+ujSTIb+OlMSJUWV4opS7WVNnxHbFTPYZVQ3erv7NKc2iVizCRZ2Kxn srM1oPXWRic8BIAdYOKOloF2300SL/bIpeD+x7h3w9B/qez7nOin5NzkxgFoaUeIal12pXSR Q354FKFoy6Vh96gc4VRqte3jw8mPuJQpfws+Pb+swvSf/i1q1+1I4jsRQQh2m6OTADHIqg2E ofTYAEh7R5HfPx0EXoEDMdRjOeKn8+vvkAwhviWXTHlG3R1QkbE5M/oywnZ83udJmi+lxjJ5 YhQ5IzomvJ16H0Bq+TLyVLO/VRksp1VR9HxCzItLNCS8PdpYYz5TC204ViycobYU65WMpzWe LFAGn8jSS25XIpqv0Y9k87dLbctKKA14Ifw2kq5OIVu2FuX+3i446JOa2vpCI9GcjCzi3oHV e00bzYiHMIl0FICrNJU0Kjho8pdo0m2uxkn6SYEpogAy9pnatUlO+erL4LqFUO7GXSdBRbw5 gNt25XTLdSFuZtMxkY3tq8MFss5QnjhehCVPEpE6y9ZjI4XB8ad1G4oBHVGK5LMsvg22PfMJ ISWFSHoF/B5+lHkCKWkFxZ0gZn33ju5n6/FOdEx4B8cMJt+cWwARAQABzSlBbmRyZXcgQ29v cGVyIDxhbmRyZXcuY29vcGVyM0BjaXRyaXguY29tPsLBegQTAQgAJAIbAwULCQgHAwUVCgkI CwUWAgMBAAIeAQIXgAUCWKD95wIZAQAKCRBlw/kGpdefoHbdD/9AIoR3k6fKl+RFiFpyAhvO 59ttDFI7nIAnlYngev2XUR3acFElJATHSDO0ju+hqWqAb8kVijXLops0gOfqt3VPZq9cuHlh IMDquatGLzAadfFx2eQYIYT+FYuMoPZy/aTUazmJIDVxP7L383grjIkn+7tAv+qeDfE+txL4 SAm1UHNvmdfgL2/lcmL3xRh7sub3nJilM93RWX1Pe5LBSDXO45uzCGEdst6uSlzYR/MEr+5Z JQQ32JV64zwvf/aKaagSQSQMYNX9JFgfZ3TKWC1KJQbX5ssoX/5hNLqxMcZV3TN7kU8I3kjK mPec9+1nECOjjJSO/h4P0sBZyIUGfguwzhEeGf4sMCuSEM4xjCnwiBwftR17sr0spYcOpqET ZGcAmyYcNjy6CYadNCnfR40vhhWuCfNCBzWnUW0lFoo12wb0YnzoOLjvfD6OL3JjIUJNOmJy RCsJ5IA/Iz33RhSVRmROu+TztwuThClw63g7+hoyewv7BemKyuU6FTVhjjW+XUWmS/FzknSi dAG+insr0746cTPpSkGl3KAXeWDGJzve7/SBBfyznWCMGaf8E2P1oOdIZRxHgWj0zNr1+ooF /PzgLPiCI4OMUttTlEKChgbUTQ+5o0P080JojqfXwbPAyumbaYcQNiH1/xYbJdOFSiBv9rpt TQTBLzDKXok86M7BTQRS4TZ/ARAAkgqudHsp+hd82UVkvgnlqZjzz2vyrYfz7bkPtXaGb9H4 Rfo7mQsEQavEBdWWjbga6eMnDqtu+FC+qeTGYebToxEyp2lKDSoAsvt8w82tIlP/EbmRbDVn 7bhjBlfRcFjVYw8uVDPptT0TV47vpoCVkTwcyb6OltJrvg/QzV9f07DJswuda1JH3/qvYu0p vjPnYvCq4NsqY2XSdAJ02HrdYPFtNyPEntu1n1KK+gJrstjtw7KsZ4ygXYrsm/oCBiVW/OgU g/XIlGErkrxe4vQvJyVwg6YH653YTX5hLLUEL1NS4TCo47RP+wi6y+TnuAL36UtK/uFyEuPy wwrDVcC4cIFhYSfsO0BumEI65yu7a8aHbGfq2lW251UcoU48Z27ZUUZd2Dr6O/n8poQHbaTd 6bJJSjzGGHZVbRP9UQ3lkmkmc0+XCHmj5WhwNNYjgbbmML7y0fsJT5RgvefAIFfHBg7fTY/i kBEimoUsTEQz+N4hbKwo1hULfVxDJStE4sbPhjbsPCrlXf6W9CxSyQ0qmZ2bXsLQYRj2xqd1 bpA+1o1j2N4/au1R/uSiUFjewJdT/LX1EklKDcQwpk06Af/N7VZtSfEJeRV04unbsKVXWZAk uAJyDDKN99ziC0Wz5kcPyVD1HNf8bgaqGDzrv3TfYjwqayRFcMf7xJaL9xXedMcAEQEAAcLB XwQYAQgACQUCUuE2fwIbDAAKCRBlw/kGpdefoG4XEACD1Qf/er8EA7g23HMxYWd3FXHThrVQ HgiGdk5Yh632vjOm9L4sd/GCEACVQKjsu98e8o3ysitFlznEns5EAAXEbITrgKWXDDUWGYxd pnjj2u+GkVdsOAGk0kxczX6s+VRBhpbBI2PWnOsRJgU2n10PZ3mZD4Xu9kU2IXYmuW+e5KCA vTArRUdCrAtIa1k01sPipPPw6dfxx2e5asy21YOytzxuWFfJTGnVxZZSCyLUO83sh6OZhJkk b9rxL9wPmpN/t2IPaEKoAc0FTQZS36wAMOXkBh24PQ9gaLJvfPKpNzGD8XWR5HHF0NLIJhgg 4ZlEXQ2fVp3XrtocHqhu4UZR4koCijgB8sB7Tb0GCpwK+C4UePdFLfhKyRdSXuvY3AHJd4CP 4JzW0Bzq/WXY3XMOzUTYApGQpnUpdOmuQSfpV9MQO+/jo7r6yPbxT7CwRS5dcQPzUiuHLK9i nvjREdh84qycnx0/6dDroYhp0DFv4udxuAvt1h4wGwTPRQZerSm4xaYegEFusyhbZrI0U9tJ B8WrhBLXDiYlyJT6zOV2yZFuW47VrLsjYnHwn27hmxTC/7tvG3euCklmkn9Sl9IAKFu29RSo d5bD8kMSCYsTqtTfT6W4A3qHGvIDta3ptLYpIAOD2sY3GYq2nf3Bbzx81wZK14JdDDHUX2Rs 6+ahAA== In-Reply-To: Content-Type: text/plain; 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> +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; This means one (or more) subsystem has brk_alloc()'d more than they reserved. While returning NULL is probably the best action, I think a printk_once() is also warranted. > + > + return ptr; > +} > + > +unsigned long __init brk_get_unused_start(void) > +{ > + finished = true; This doesn't get the unused start.  It also terminates the allocator, and the name needs to reflect that. But combined with brk_end in the next patch, it's really quite a mess.  Integrating brk_end properly simplifies this patch too. > + > + 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 > + > +void *brk_alloc(size_t size); > +unsigned long brk_get_unused_start(void); > +void brk_free_unused(void); It's inevitable that brk is going to be used in other architectures, and none of this is x86 specific. This should be xen/include/brk.h right from the outset, even if that's all we do in the way of making it generic. Except, brk_end in patch 2 really needs to live in common/brk.c so it's probably easier to go in arch-neutral right from the outset and use "select ARCH_HAS_BRK" or so to allow the arches to opt into using it. Furthermore, this header needs some commentary.  To do this nicely, DEFINE_BRK() needs to be introduced here so the comment makes sense. ----- Early Boot memory allocator. Subsystems which conditionally need memory prior to the main heap being set up should use DEFINE_BRK() to reserve BSS space in Xen. During boot, brk_alloc() allocates memory from the reserved space.  Such allocations are good for the lifetime of Xen.  Subsystems MUST NOT brk_alloc() more memory than they reserved. When the main heap is set up, brk allocations become unavailable.  Reserved but unallocated space is handed to the main heap, so it doesn't go to waste. ----- With just a few sentences, it's now far clearer what brk is and how to use it. > --- a/xen/arch/x86/xen.lds.S > +++ b/xen/arch/x86/xen.lds.S > @@ -321,7 +321,11 @@ 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); This looks fine, but if it's becoming common then it wants to be a macro in xen.lds.h ~Andrew