Re: [patch] paravirt: VDSO page is essential

Rusty Russell
2007-03-06T00:58:28Z

On Tue, 2007-03-06 at 00:28 +1100, Rusty Russell wrote:
> On Mon, 2007-03-05 at 13:06 +0100, Ingo Molnar wrote:
> > Subject: [patch] paravirt: VDSO page is essential
> > From: Ingo Molnar <mingo@elte.hu>
> >
> > commit 3bbf54725467d604698721384d858b5983b87e8f disables the VDSO for
> > CONFIG_PARAVIRT kernels. This #ifdeffery was a bad change: the VDSO is
> > an essential component of Linux, and this change forces all of them to
> > use int $0x80 - including sane ones like KVM. (If a hypervisor does not
> > handle the VDSO properly then it can work things around via the vdso=0
> > boot option. Or CONFIG_PARAVIRT should not have been merged. But in any
> > case, it is a basic taste issue: we DO NOT #ifdef around core features
> > like this!)
>
> I agree with the criticism, dislike the snarly comments, and disagree
> with this patch.

And my patch was pretty crack-induced too. Sorry.

I shouldn't have been thinking about using CONFIG options at all: we
should simply disable the vdso if CONFIG_COMPAT_VDSO=y when we
*actually* reserve top memory.

This still need some work (doing that now), but do people like the idea?

The current "vdso_disabled" flag merely disabled the ELF note, so it
needs to be made a little stronger, to not set up the vdso at all.

diff -r f75715e64a3b arch/i386/Kconfig
--- a/arch/i386/Kconfig Tue Mar 06 00:04:50 2007 +1100
+++ b/arch/i386/Kconfig Tue Mar 06 09:30:36 2007 +1100
@@ -893,9 +893,10 @@ config COMPAT_VDSO
config COMPAT_VDSO
bool "Compat VDSO support"
default y
- depends on !PARAVIRT
- help
- Map the VDSO to the predictable old-style address too.
+ help
+ Map the VDSO to the predictable old-style address too, or
+ in the case of a VMI/Xen/lguest virtualized guest, don't create
+ the VDSO at all.
---help---
Say N here if you are running a sufficiently recent glibc
version (2.3.3 or later), to remove the high-mapped
diff -r f75715e64a3b arch/i386/kernel/sysenter.c
--- a/arch/i386/kernel/sysenter.c Tue Mar 06 00:04:50 2007 +1100
+++ b/arch/i386/kernel/sysenter.c Tue Mar 06 09:25:47 2007 +1100
@@ -27,11 +27,7 @@
* Should the kernel map a VDSO page into processes and pass its
* address down to glibc upon exec()?
*/
-#ifdef CONFIG_PARAVIRT
-unsigned int __read_mostly vdso_enabled = 0;
-#else
unsigned int __read_mostly vdso_enabled = 1;
-#endif

EXPORT_SYMBOL_GPL(vdso_enabled);

@@ -51,7 +47,7 @@ void enable_sep_cpu(void)
int cpu = get_cpu();
struct tss_struct *tss = &per_cpu(init_tss, cpu);

- if (!boot_cpu_has(X86_FEATURE_SEP)) {
+ if (!boot_cpu_has(X86_FEATURE_SEP) || !vdso_enabled) {
put_cpu();
return;
}
@@ -74,7 +70,12 @@ static struct page *syscall_pages[1];

int __init sysenter_setup(void)
{
- void *syscall_page = (void *)get_zeroed_page(GFP_ATOMIC);
+ void *syscall_page;
+
+ if (!vdso_enabled)
+ return 0;
+
+ syscall_page = (void *)get_zeroed_page(GFP_ATOMIC);
syscall_pages[0] = virt_to_page(syscall_page);

#ifdef CONFIG_COMPAT_VDSO
@@ -106,6 +107,9 @@ int arch_setup_additional_pages(struct l
struct mm_struct *mm = current->mm;
unsigned long addr;
int ret;
+
+ if (!vdso_enabled)
+ return 0;

down_write(&mm->mmap_sem);
addr = get_unmapped_area(NULL, 0, PAGE_SIZE, 0, 0);
diff -r f75715e64a3b arch/i386/mm/pgtable.c
--- a/arch/i386/mm/pgtable.c Tue Mar 06 00:04:50 2007 +1100
+++ b/arch/i386/mm/pgtable.c Tue Mar 06 09:32:51 2007 +1100
@@ -144,10 +144,8 @@ void set_pmd_pfn(unsigned long vaddr, un
}

static int fixmaps;
-#ifndef CONFIG_COMPAT_VDSO
unsigned long __FIXADDR_TOP = 0xfffff000;
EXPORT_SYMBOL(__FIXADDR_TOP);
-#endif

void __set_fixmap (enum fixed_addresses idx, unsigned long phys, pgprot_t flags)
{
@@ -174,11 +172,12 @@ void reserve_top_address(unsigned long r
printk(KERN_INFO "Reserving virtual address space above 0x%08x\n",
(int)-reserve);
#ifdef CONFIG_COMPAT_VDSO
- BUG_ON(reserve != 0);
-#else
+ /* We can't have both reserved space and VDSO at 0xFFFFE000. */
+ if (reserve)
+ vdso_enabled = 0;
+#endif
__FIXADDR_TOP = -reserve - PAGE_SIZE;
__VMALLOC_RESERVE += reserve;
-#endif
}

pte_t *pte_alloc_one_kernel(struct mm_struct *mm, unsigned long address)
diff -r f75715e64a3b include/asm-i386/fixmap.h
--- a/include/asm-i386/fixmap.h Tue Mar 06 00:04:50 2007 +1100
+++ b/include/asm-i386/fixmap.h Tue Mar 06 09:29:15 2007 +1100
@@ -19,10 +19,8 @@
* Leave one empty page between vmalloc'ed areas and
* the start of the fixmap.
*/
-#ifndef CONFIG_COMPAT_VDSO
extern unsigned long __FIXADDR_TOP;
-#else
-#define __FIXADDR_TOP 0xfffff000
+#ifdef CONFIG_COMPAT_VDSO
#define FIXADDR_USER_START __fix_to_virt(FIX_VDSO)
#define FIXADDR_USER_END __fix_to_virt(FIX_VDSO - 1)
#endif