Re: [patch 8/9] F00F bug fixup for i386 - use conditional calls

Andrew Morton
2007-05-30T20:35:00Z

On Wed, 30 May 2007 10:00:33 -0400
Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca> wrote:

> Use the faster conditional calls for F00F bug handling in do_page_fault.
>

I guess this means that CONDCALL will be enabled on pretty much all i386,
in which case making the whole feature Kconfigurable is starting to look
marginal.

Perhaps a better approach would have to made this change dependent upon
CONDCALL, rather than forcing it on.

> @@ -1084,6 +1085,7 @@
> */
> idt_descr.address = fix_to_virt(FIX_F00F_IDT);
> load_idt(&idt_descr);
> + BUG_ON(cond_call_arm("fix_f00f"));

It is generally poor C style to do

assert(something_which_has_side_effects())

because people can legitimately expect to do

#define assert() /*nothing*/

for production code.

The kernel doesn't actually do the right thing here when CONFIG_BUG=n,
sadly. But still, children might be watching, so the better and preferred
style is

if (cond_call_arm("fix_f00f"))
BUG();

> }
> #endif
>
> Index: linux-2.6-lttng/arch/i386/mm/fault.c
> ===================================================================
> --- linux-2.6-lttng.orig/arch/i386/mm/fault.c 2007-05-29 11:05:48.000000000 -0400
> +++ linux-2.6-lttng/arch/i386/mm/fault.c 2007-05-29 11:13:16.000000000 -0400
> @@ -25,6 +25,7 @@
> #include <linux/kprobes.h>
> #include <linux/uaccess.h>
> #include <linux/kdebug.h>
> +#include <linux/condcall.h>
>
> #include <asm/system.h>
> #include <asm/desc.h>
> @@ -221,6 +222,25 @@
>
> fastcall void do_invalid_op(struct pt_regs *, unsigned long);
>
> +#ifdef CONFIG_X86_F00F_BUG
> +/*
> + * Pentium F0 0F C7 C8 bug workaround.
> + */
> +static inline int do_f00f_workaround(struct pt_regs *regs,
> + unsigned long address)
> +{
> + unsigned long nr;
> +
> + nr = (address - idt_descr.address) >> 3;
> +
> + if (nr == 6) {
> + do_invalid_op(regs, 0);
> + return 1;
> + }
> + return 0;
> +}
> +#endif
> +
> static inline pmd_t *vmalloc_sync_one(pgd_t *pgd, unsigned long address)
> {
> unsigned index = pgd_index(address);
> @@ -474,19 +494,8 @@
> }
>
> #ifdef CONFIG_X86_F00F_BUG
> - /*
> - * Pentium F0 0F C7 C8 bug workaround.
> - */
> - if (boot_cpu_data.f00f_bug) {
> - unsigned long nr;
> -
> - nr = (address - idt_descr.address) >> 3;
> -
> - if (nr == 6) {
> - do_invalid_op(regs, 0);
> - return;
> - }
> - }
> + if (cond_call(fix_f00f, do_f00f_workaround(regs, address)))
> + return;

We do a cond_call() to an inlined function? That's a bit weird, isn't it?

> #endif
>
> no_context:
> Index: linux-2.6-lttng/arch/i386/Kconfig.cpu
> ===================================================================
> --- linux-2.6-lttng.orig/arch/i386/Kconfig.cpu 2007-05-29 11:51:46.000000000 -0400
> +++ linux-2.6-lttng/arch/i386/Kconfig.cpu 2007-05-29 11:52:08.000000000 -0400
> @@ -275,6 +275,7 @@
> config X86_F00F_BUG
> bool
> depends on M586MMX || M586TSC || M586 || M486 || M386
> + select COND_CALL

That hurts.