From 4f2c223c59eb25fe8e9e3632a78541a1fddaa921 Mon Sep 17 00:00:00 2001 From: Jason Andryuk Date: Thu, 11 Jun 2026 09:02:18 +0200 Subject: [PATCH] xen/x86: Change stub page allocation/free MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Today the inline tracking of the stub page is problematic. 0xcc is used to indicate unused, but it is also a "clear value." A !CONFIG_PV build or when running with FRED support will not populate the LSTAR/CSTAR stubs at CPU bringup. If a CPU is then offlined, the stubs page will be freed as its content will be all 0xcc, regardless of the stubs page still begin referenced by other CPUs. The new approach uses a global, CPU-indexed dynamically allocated array of stub addresses. However, to handle NUMA aware allocations, we cannot allocate all the memory in advance because of the NUMA dependency. Take advantage of the fact that Xen will attempt to contiguously pack CPUs on the same NUMA node (see normalise_cpu_order()), and on CPU bringup use the same stubs page the previous CPU did if suitable. Note the code would still function properly even if CPUs from NUMA nodes are not contiguously packed, it just consumes more memory. Stubs pages are no longer freed. They remain referenced in the global CPU-indexed array and are re-used if the CPU is re-onlined. The stubs array doesn't have an explicit lock. During boot it's accessed single threaded. During runtime, &cpu_add_remove_lock serializes access. Fixes: 7a66ac8d1633 ("x86: move syscall trampolines off the stack") Signed-off-by: Jason Andryuk Signed-off-by: Roger Pau Monné Tested-by: Jason Andryuk Reviewed-by: Jason Andryuk Reviewed-by: Jan Beulich Reviewed-by: Andrew Cooper Release-Acked-by: Oleksii Kurochko --- xen/arch/x86/include/asm/stubs.h | 2 +- xen/arch/x86/setup.c | 4 +- xen/arch/x86/smpboot.c | 94 +++++++++++++++++--------------- 3 files changed, 52 insertions(+), 48 deletions(-) diff --git a/xen/arch/x86/include/asm/stubs.h b/xen/arch/x86/include/asm/stubs.h index a520928e9a..6f843a0f48 100644 --- a/xen/arch/x86/include/asm/stubs.h +++ b/xen/arch/x86/include/asm/stubs.h @@ -32,6 +32,6 @@ struct stubs { }; DECLARE_PER_CPU(struct stubs, stubs); -unsigned long alloc_stub_page(unsigned int cpu, unsigned long *mfn); +void init_stubs(void); #endif /* X86_ASM_STUBS_H */ diff --git a/xen/arch/x86/setup.c b/xen/arch/x86/setup.c index 4192edf635..7d71fea6c0 100644 --- a/xen/arch/x86/setup.c +++ b/xen/arch/x86/setup.c @@ -2089,9 +2089,7 @@ void asmlinkage __init noreturn __start_xen(void) init_idle_domain(); - this_cpu(stubs.addr) = alloc_stub_page(smp_processor_id(), - &this_cpu(stubs).mfn); - BUG_ON(!this_cpu(stubs.addr)); + init_stubs(); bsp_traps_reinit(); /* Needs stubs allocated, must be before presmp_initcalls. */ diff --git a/xen/arch/x86/smpboot.c b/xen/arch/x86/smpboot.c index d8fd71ffab..84e9e4beed 100644 --- a/xen/arch/x86/smpboot.c +++ b/xen/arch/x86/smpboot.c @@ -20,6 +20,7 @@ #include #include #include +#include #include #include @@ -641,41 +642,64 @@ static int do_boot_cpu(int apicid, int cpu) return rc; } -#define STUB_BUF_CPU_OFFS(cpu) (((cpu) & (STUBS_PER_PAGE - 1)) * STUB_BUF_SIZE) +/* Dynamically allocated, indexed by CPU. Store physical address of stubs. */ +static paddr_t *__ro_after_init stubs; -unsigned long alloc_stub_page(unsigned int cpu, unsigned long *mfn) +static bool assign_stub_page(unsigned int cpu) { unsigned long stub_va; - struct page_info *pg; + paddr_t addr = stubs[cpu]; - BUILD_BUG_ON(STUBS_PER_PAGE & (STUBS_PER_PAGE - 1)); - - if ( *mfn ) - pg = mfn_to_page(_mfn(*mfn)); - else + if ( addr == INVALID_PADDR ) { - nodeid_t node = cpu_to_node(cpu); - unsigned int memflags = node != NUMA_NO_NODE ? MEMF_node(node) : 0; + nodeid_t nid = cpu_to_node(cpu); - pg = alloc_domheap_page(NULL, memflags); - if ( !pg ) - return 0; + /* + * Attempt to use the same page as the previous CPU if possible, + * otherwise allocate a new one. + */ + if ( cpu && nid == cpu_to_node(cpu - 1) && + stubs[cpu - 1] != INVALID_PADDR && + PAGE_OFFSET(stubs[cpu - 1] + STUB_BUF_SIZE) ) + addr = stubs[cpu - 1] + STUB_BUF_SIZE; + else + { + struct page_info *pg = alloc_domheap_page(NULL, MEMF_node(nid)); - unmap_domain_page(memset(__map_domain_page(pg), 0xcc, PAGE_SIZE)); + if ( !pg ) + return false; + + unmap_domain_page(memset(__map_domain_page(pg), 0xcc, PAGE_SIZE)); + addr = page_to_maddr(pg); + } + stubs[cpu] = addr; } stub_va = XEN_VIRT_END - FIXADDR_X_SIZE - (cpu + 1) * PAGE_SIZE; - if ( map_pages_to_xen(stub_va, page_to_mfn(pg), 1, + if ( map_pages_to_xen(stub_va, maddr_to_mfn(addr), 1, PAGE_HYPERVISOR_RX | MAP_SMALL_PAGES) ) - { - if ( !*mfn ) - free_domheap_page(pg); - stub_va = 0; - } - else if ( !*mfn ) - *mfn = mfn_x(page_to_mfn(pg)); + return false; - return stub_va; + per_cpu(stubs.mfn, cpu) = PFN_DOWN(addr); + per_cpu(stubs.addr, cpu) = stub_va + PAGE_OFFSET(addr); + return true; +} + +void __init init_stubs(void) +{ + const unsigned int num_cpus = num_present_cpus(); + unsigned int i; + + ASSERT(!stubs); + stubs = xvmalloc_array(typeof(*stubs), num_cpus); + if ( !stubs ) + panic("Unable to allocate stub array\n"); + + for ( i = 0; i < num_cpus; i++ ) + stubs[i] = INVALID_PADDR; + + if ( !assign_stub_page(0) ) + panic("Unable to initialize BSP stub region\n"); } void cpu_exit_clear(unsigned int cpu) @@ -990,19 +1014,12 @@ static void cpu_smpboot_free(unsigned int cpu, bool remove) { mfn_t mfn = _mfn(per_cpu(stubs.mfn, cpu)); unsigned char *stub_page = map_domain_page(mfn); - unsigned int i; - memset(stub_page + STUB_BUF_CPU_OFFS(cpu), 0xcc, STUB_BUF_SIZE); - for ( i = 0; i < STUBS_PER_PAGE; ++i ) - if ( stub_page[i * STUB_BUF_SIZE] != 0xcc ) - break; + memset(stub_page + PAGE_OFFSET(stubs[cpu]), 0xcc, STUB_BUF_SIZE); unmap_domain_page(stub_page); destroy_xen_mappings(per_cpu(stubs.addr, cpu) & PAGE_MASK, (per_cpu(stubs.addr, cpu) | ~PAGE_MASK) + 1); per_cpu(stubs.addr, cpu) = 0; - per_cpu(stubs.mfn, cpu) = 0; - if ( i == STUBS_PER_PAGE ) - free_domheap_page(mfn_to_page(mfn)); } if ( IS_ENABLED(CONFIG_PV32) ) @@ -1041,10 +1058,9 @@ void *cpu_alloc_stack(unsigned int cpu) static int cpu_smpboot_alloc(unsigned int cpu) { struct cpu_info *info; - unsigned int i, memflags = 0; + unsigned int memflags = 0; nodeid_t node = cpu_to_node(cpu); seg_desc_t *gdt; - unsigned long stub_page; int rc = -ENOMEM; if ( node != NUMA_NO_NODE ) @@ -1092,18 +1108,8 @@ static int cpu_smpboot_alloc(unsigned int cpu) memcpy(per_cpu(idt, cpu), bsp_idt, sizeof(bsp_idt)); disable_each_ist(per_cpu(idt, cpu)); - for ( stub_page = 0, i = cpu & ~(STUBS_PER_PAGE - 1); - i < nr_cpu_ids && i <= (cpu | (STUBS_PER_PAGE - 1)); ++i ) - if ( cpu_online(i) && cpu_to_node(i) == node ) - { - per_cpu(stubs.mfn, cpu) = per_cpu(stubs.mfn, i); - break; - } - BUG_ON(i == cpu); - stub_page = alloc_stub_page(cpu, &per_cpu(stubs.mfn, cpu)); - if ( !stub_page ) + if ( !assign_stub_page(cpu) ) goto out; - per_cpu(stubs.addr, cpu) = stub_page + STUB_BUF_CPU_OFFS(cpu); rc = setup_cpu_root_pgt(cpu); if ( rc )