Re: [RFC, PATCH 7/24] i386 Vmi memory hole

From: Zachary Amsden
Date: Tue Mar 14 2006 - 02:14:24 EST

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Chris Wright wrote:

* Zachary Amsden (zach@xxxxxxxxxx) wrote:

Create a configurable hole in the linear address space at the top
of memory. A more advanced interface is needed to negotiate how
much space the hypervisor is allowed to steal, but in the end, it
seems most likely that a fixed constant size will be chosen for
the compiled kernel, potentially propagated to an information
page used by paravirtual initialization to determine interface
compatibility.

Signed-off-by: Zachary Amsden <zach@xxxxxxxxxx>

Index: linux-2.6.16-rc3/arch/i386/Kconfig
===================================================================
--- linux-2.6.16-rc3.orig/arch/i386/Kconfig 2006-02-22 16:09:04.000000000 -0800
+++ linux-2.6.16-rc3/arch/i386/Kconfig 2006-02-22 16:33:27.000000000 -0800
@@ -201,6 +201,15 @@ config VMI_DEBUG
endmenu
+config MEMORY_HOLE
+ int "Create hole at top of memory (0-256 MB)"
+ range 0 256
+ default "64" if X86_VMI
+ default "0" if !X86_VMI

Deja-vu ;-) And still works in context of Xen, but we've just let the
subarch define the __FIXADDR_TOP. Having it be dynamic could be
interesting.

Here's dynamic. I hope it still applies. Allow creation of an compile time hole at the top of linear address space.

Extended to allow a dynamic hole in linear address space, 7/2005. This
required some serious hacking to get everything perfect, but the end result
appears to function quite nicely. Everyone can now share the appreciation
of pseudo-undocumented ELF OS fields, which means core dumps, debuggers
and even broken or obsolete linkers may continue to work.

Signed-off-by: Zachary Amsden <zach@xxxxxxxxxx>
Index: linux-2.6.13/arch/i386/Kconfig
===================================================================
--- linux-2.6.13.orig/arch/i386/Kconfig 2005-08-04 14:14:24.000000000 -0700
+++ linux-2.6.13/arch/i386/Kconfig 2005-08-05 15:28:42.000000000 -0700
@@ -127,6 +127,20 @@

endchoice

+config RELOCATABLE_FIXMAP
+ bool "Allow the fixmap to be placed dynamically at runtime"
+ depends on EXPERIMENTAL
+ help
+ Crazy hackers only.
+
+config MEMORY_HOLE
+ int "Create hole at top of memory (0-512 MB)"
+ range 0 512
+ default "0"
+ help
+ Useful for creating a hole in the top of memory when running
+ inside of a virtual machine monitor.
+
config ACPI_SRAT
bool
default y
Index: linux-2.6.13/arch/i386/kernel/sysenter.c
===================================================================
--- linux-2.6.13.orig/arch/i386/kernel/sysenter.c 2005-08-02 17:04:12.000000000 -0700
+++ linux-2.6.13/arch/i386/kernel/sysenter.c 2005-08-05 15:47:53.000000000 -0700
@@ -46,22 +46,90 @@
extern const char vsyscall_int80_start, vsyscall_int80_end;
extern const char vsyscall_sysenter_start, vsyscall_sysenter_end;

+#ifdef CONFIG_RELOCATABLE_FIXMAP
+extern const char SYSENTER_RETURN;
+const char *SYSENTER_RETURN_ADDR;
+
+static void fixup_vsyscall_elf(char *page)
+{
+ Elf32_Ehdr *hdr;
+ Elf32_Shdr *sechdrs;
+ Elf32_Phdr *phdr;
+ char *secstrings;
+ int i, j, n;
+
+ hdr = (Elf32_Ehdr *)page;
+
+ /* Sanity checks against insmoding binaries or wrong arch,
+ weird elf version */
+ if (memcmp(hdr->e_ident, ELFMAG, 4) != 0 ||
+ !elf_check_arch(hdr) ||
+ hdr->e_type != ET_DYN)
+ panic("Bogus ELF in vsyscall DSO\n");
+
+ hdr->e_entry += VSYSCALL_RELOCATION;
+
+ sechdrs = (void *)hdr + hdr->e_shoff;
+ secstrings = (void *)hdr + sechdrs[hdr->e_shstrndx].sh_offset;
+
+ for (i = 1; i < hdr->e_shnum; i++) {
+ if (!(sechdrs[i].sh_flags & SHF_ALLOC))
+ continue;
+
+ sechdrs[i].sh_addr += VSYSCALL_RELOCATION;
+ if (strcmp(secstrings+sechdrs[i].sh_name, ".dynsym") == 0) {
+ Elf32_Sym *sym = (void *)hdr + sechdrs[i].sh_offset;
+ n = sechdrs[i].sh_size / sizeof(*sym);
+ for (j = 1; j < n; j++) {
+ int ndx = sym[j].st_shndx;
+ if (ndx == SHN_UNDEF || ndx == SHN_ABS)
+ continue;
+ sym[j].st_value += VSYSCALL_RELOCATION;
+ }
+ } else if (strcmp(secstrings+sechdrs[i].sh_name, ".dynamic") == 0) {
+ Elf32_Dyn *dyn = (void *)hdr + sechdrs[i].sh_offset;
+ int tag;
+ while ((tag = (++dyn)->d_tag) != DT_NULL) {
+ if (tag == DT_PLTGOT || tag == DT_HASH ||
+ tag == DT_STRTAB || tag == DT_SYMTAB ||
+ tag == DT_RELA || tag == DT_INIT ||
+ tag == DT_FINI || tag == DT_REL ||
+ tag == DT_JMPREL || tag == DT_VERSYM ||
+ tag == DT_VERDEF || tag == DT_VERNEED)
+ dyn->d_un.d_val += VSYSCALL_RELOCATION;
+ }
+ } else if (strcmp(secstrings+sechdrs[i].sh_name, ".useless") == 0) {
+ uint32_t *got = (void *)hdr + sechdrs[i].sh_offset;
+ *got += VSYSCALL_RELOCATION;
+ }
+ }
+ phdr = (void *)hdr + hdr->e_phoff;
+ for (i = 0; i < hdr->e_phnum; i++) {
+ phdr[i].p_vaddr += VSYSCALL_RELOCATION;
+ phdr[i].p_paddr += VSYSCALL_RELOCATION;
+ }
+ SYSENTER_RETURN_ADDR = (char *)&SYSENTER_RETURN + VSYSCALL_RELOCATION;
+}
+#endif
+
int __init sysenter_setup(void)
{
void *page = (void *)get_zeroed_page(GFP_ATOMIC);

- __set_fixmap(FIX_VSYSCALL, __pa(page), PAGE_READONLY_EXEC);
-
- if (!boot_cpu_has(X86_FEATURE_SEP)) {
+ if (!boot_cpu_has(X86_FEATURE_SEP))
memcpy(page,
&vsyscall_int80_start,
&vsyscall_int80_end - &vsyscall_int80_start);
- return 0;
- }
+ else
+ memcpy(page,
+ &vsyscall_sysenter_start,
+ &vsyscall_sysenter_end - &vsyscall_sysenter_start);

- memcpy(page,
- &vsyscall_sysenter_start,
- &vsyscall_sysenter_end - &vsyscall_sysenter_start);
+#ifdef CONFIG_RELOCATABLE_FIXMAP
+ fixup_vsyscall_elf((char *)page);
+#endif
+
+ __set_fixmap(FIX_VSYSCALL, __pa(page), PAGE_READONLY_EXEC);

return 0;
}
Index: linux-2.6.13/arch/i386/kernel/asm-offsets.c
===================================================================
--- linux-2.6.13.orig/arch/i386/kernel/asm-offsets.c 2005-08-04 14:28:35.000000000 -0700
+++ linux-2.6.13/arch/i386/kernel/asm-offsets.c 2005-08-05 15:11:45.000000000 -0700
@@ -68,5 +68,9 @@
sizeof(struct tss_struct));

DEFINE(PAGE_SIZE_asm, PAGE_SIZE);
+#ifdef CONFIG_RELOCATABLE_FIXMAP
+ DEFINE(VSYSCALL_BASE, 0);
+#else
DEFINE(VSYSCALL_BASE, __fix_to_virt(FIX_VSYSCALL));
+#endif
}
Index: linux-2.6.13/arch/i386/kernel/signal.c
===================================================================
--- linux-2.6.13.orig/arch/i386/kernel/signal.c 2005-08-03 23:36:46.000000000 -0700
+++ linux-2.6.13/arch/i386/kernel/signal.c 2005-08-05 15:11:33.000000000 -0700
@@ -345,6 +345,8 @@
See vsyscall-sigreturn.S. */
extern void __user __kernel_sigreturn;
extern void __user __kernel_rt_sigreturn;
+#define kernel_sigreturn (VSYSCALL_RELOCATION + (void __user *)&__kernel_sigreturn)
+#define kernel_rt_sigreturn (VSYSCALL_RELOCATION + (void __user *)&__kernel_rt_sigreturn)

static int setup_frame(int sig, struct k_sigaction *ka,
sigset_t *set, struct pt_regs * regs)
@@ -380,7 +382,7 @@
goto give_sigsegv;
}

- restorer = &__kernel_sigreturn;
+ restorer = kernel_sigreturn;
if (ka->sa.sa_flags & SA_RESTORER)
restorer = ka->sa.sa_restorer;

@@ -476,7 +478,7 @@
goto give_sigsegv;

/* Set up to return from userspace. */
- restorer = &__kernel_rt_sigreturn;
+ restorer = kernel_rt_sigreturn;
if (ka->sa.sa_flags & SA_RESTORER)
restorer = ka->sa.sa_restorer;
err |= __put_user(restorer, &frame->pretcode);
Index: linux-2.6.13/arch/i386/kernel/entry.S
===================================================================
--- linux-2.6.13.orig/arch/i386/kernel/entry.S 2005-08-04 14:17:15.000000000 -0700
+++ linux-2.6.13/arch/i386/kernel/entry.S 2005-08-05 14:09:15.000000000 -0700
@@ -200,7 +200,11 @@
pushl %ebp
pushfl
pushl $(__USER_CS)
+#ifdef CONFIG_RELOCATABLE_FIXMAP
+ pushl %ss:SYSENTER_RETURN_ADDR
+#else
pushl $SYSENTER_RETURN
+#endif

/*
* Load the potential sixth argument from user stack.
Index: linux-2.6.13/arch/i386/mm/init.c
===================================================================
--- linux-2.6.13.orig/arch/i386/mm/init.c 2005-08-04 14:39:17.000000000 -0700
+++ linux-2.6.13/arch/i386/mm/init.c 2005-08-05 15:20:04.000000000 -0700
@@ -42,6 +42,10 @@

unsigned int __VMALLOC_RESERVE = 128 << 20;

+#ifdef CONFIG_RELOCATABLE_FIXMAP
+unsigned long __FIXADDR_TOP = 0;
+#endif
+
DEFINE_PER_CPU(struct mmu_gather, mmu_gathers);
unsigned long highstart_pfn, highend_pfn;

@@ -478,6 +482,12 @@
printk("NX (Execute Disable) protection: active\n");
#endif

+#ifdef CONFIG_RELOCATABLE_FIXMAP
+ if (!__FIXADDR_TOP)
+ __FIXADDR_TOP = 0xfffff000UL-(CONFIG_MEMORY_HOLE << 20);
+ printk(KERN_INFO "Fixmap top relocated to %lxh\n", __FIXADDR_TOP);
+#endif
+
pagetable_init();

load_cr3(swapper_pg_dir);
Index: linux-2.6.13/include/asm-i386/fixmap.h
===================================================================
--- linux-2.6.13.orig/include/asm-i386/fixmap.h 2005-08-04 14:14:24.000000000 -0700
+++ linux-2.6.13/include/asm-i386/fixmap.h 2005-08-05 15:36:13.000000000 -0700
@@ -20,7 +20,13 @@
* Leave one empty page between vmalloc'ed areas and
* the start of the fixmap.
*/
-#define __FIXADDR_TOP 0xfffff000
+#ifdef CONFIG_RELOCATABLE_FIXMAP
+extern unsigned long __FIXADDR_TOP;
+#define VSYSCALL_RELOCATION __fix_to_virt(FIX_VSYSCALL)
+#else
+#define __FIXADDR_TOP (0xfffff000-(CONFIG_MEMORY_HOLE << 20))
+#define VSYSCALL_RELOCATION 0
+#endif

#ifndef __ASSEMBLY__
#include <linux/kernel.h>
Index: linux-2.6.13/include/asm-i386/elf.h
===================================================================
--- linux-2.6.13.orig/include/asm-i386/elf.h 2005-08-02 17:06:23.000000000 -0700
+++ linux-2.6.13/include/asm-i386/elf.h 2005-08-05 15:31:32.000000000 -0700
@@ -129,7 +129,7 @@

#define VSYSCALL_BASE (__fix_to_virt(FIX_VSYSCALL))
#define VSYSCALL_EHDR ((const struct elfhdr *) VSYSCALL_BASE)
-#define VSYSCALL_ENTRY ((unsigned long) &__kernel_vsyscall)
+#define VSYSCALL_ENTRY ((unsigned long) (VSYSCALL_RELOCATION+&__kernel_vsyscall))
extern void __kernel_vsyscall;

#define ARCH_DLINFO \
Index: linux-2.6.13/include/linux/elf.h
===================================================================
--- linux-2.6.13.orig/include/linux/elf.h 2005-08-02 17:06:24.000000000 -0700
+++ linux-2.6.13/include/linux/elf.h 2005-08-05 12:06:17.000000000 -0700
@@ -138,6 +138,9 @@
#define DT_DEBUG 21
#define DT_TEXTREL 22
#define DT_JMPREL 23
+#define DT_VERSYM 0x6ffffff0
+#define DT_VERDEF 0x6ffffffc
+#define DT_VERNEED 0x6ffffffe
#define DT_LOPROC 0x70000000
#define DT_HIPROC 0x7fffffff

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In reply to: Chris Wright: "Re: [RFC, PATCH 7/24] i386 Vmi memory hole"
Next in thread: Chris Wright: "Re: [RFC, PATCH 7/24] i386 Vmi memory hole"
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