[v7,3/3] x86, mce: Add __mcsafe_copy()

Commit Message

Dan Williams Jan. 6, 2016, 4:37 p.m. UTC

On Tue, Jan 5, 2016 at 11:11 PM, Dan Williams <[e-mail]> wrote:
> On Tue, Jan 5, 2016 at 11:06 PM, Luck, Tony <[e-mail]> wrote:
>> You were heading towards:
>>
>> ld: undefined __mcsafe_copy
>
> True, we'd also need a dummy mcsafe_copy() definition to compile it
> out in the disabled case.
>
>> since that is also inside the #ifdef.
>>
>> Weren't you going to "select" this?
>>
>
> I do select it, but by randconfig I still need to handle the
> CONFIG_X86_MCE=n case.
>
>> I'm seriously wondering whether the ifdef still makes sense. Now I don't have an extra exception table and routines to sort/search/fixup, it doesn't seem as useful as it was a few iterations ago.
>
> Either way is ok with me.  That said, the extra definitions to allow
> it compile out when not enabled don't seem too onerous.

This works for me, because all we need is the definitions.  As long as
we don't attempt to link to mcsafe_copy() we get the benefit of
compiling this out when de-selected:


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Comments

Luck, Tony Jan. 6, 2016, 4:57 p.m. UTC | #1

>> I do select it, but by randconfig I still need to handle the
>> CONFIG_X86_MCE=n case.
>>
>>> I'm seriously wondering whether the ifdef still makes sense. Now I don't have an extra exception table and routines to sort/search/fixup, it doesn't seem as useful as it was a few iterations ago.
>>
>> Either way is ok with me.  That said, the extra definitions to allow
>> it compile out when not enabled don't seem too onerous.
>
> This works for me, because all we need is the definitions.  As long as
> we don't attempt to link to mcsafe_copy() we get the benefit of
> compiling this out when de-selected:

It seems  that Kconfig's "select" statement doesn't auto-select other things
that are dependencies of the symbol you choose.

CONFIG_MCE_KERNEL_RECOVERY really is dependent on
CONFIG_X86_MCE ... having the code for the __mcsafe_copy()
linked into the kernel won't do you any good without a machine
check handler that jumps to the fixup code.

So I think you have to select X86_MCE as well (or Kconfig needs
to be taught to do it automatically ... but I have a nagging feeling
that this is known behavior).

-Tony

Dan Williams Jan. 6, 2016, 5:05 p.m. UTC | #2

On Wed, Jan 6, 2016 at 8:57 AM, Luck, Tony <[e-mail]> wrote:
>>> I do select it, but by randconfig I still need to handle the
>>> CONFIG_X86_MCE=n case.
>>>
>>>> I'm seriously wondering whether the ifdef still makes sense. Now I don't have an extra exception table and routines to sort/search/fixup, it doesn't seem as useful as it was a few iterations ago.
>>>
>>> Either way is ok with me.  That said, the extra definitions to allow
>>> it compile out when not enabled don't seem too onerous.
>>
>> This works for me, because all we need is the definitions.  As long as
>> we don't attempt to link to mcsafe_copy() we get the benefit of
>> compiling this out when de-selected:
>
> It seems  that Kconfig's "select" statement doesn't auto-select other things
> that are dependencies of the symbol you choose.
>
> CONFIG_MCE_KERNEL_RECOVERY really is dependent on
> CONFIG_X86_MCE ... having the code for the __mcsafe_copy()
> linked into the kernel won't do you any good without a machine
> check handler that jumps to the fixup code.
>
> So I think you have to select X86_MCE as well (or Kconfig needs
> to be taught to do it automatically ... but I have a nagging feeling
> that this is known behavior).
>

I don't want to force it on, otherwise we might as well remove the
ability to configure it.  Instead I have this:

config BLK_DEV_PMEM
       select MCE_KERNEL_RECOVERY if X86_MCE && X86_64

...that way if you turn on X86_MCE and BLK_DEV_PMEM you get
MCE_KERNEL_RECOVERY by default.

Patch

diff --git a/arch/x86/include/asm/string_64.h b/arch/x86/include/asm/string_64.h
index 16a8f0e56e4a..5b24039463a4 100644
--- a/arch/x86/include/asm/string_64.h
+++ b/arch/x86/include/asm/string_64.h
@@ -78,7 +78,6 @@  int strcmp(const char *cs, const char *ct);
#define memset(s, c, n) __memset(s, c, n)
#endif

-#ifdef CONFIG_MCE_KERNEL_RECOVERY
struct mcsafe_ret {
       u64 trapnr;
       u64 remain;
@@ -86,7 +85,6 @@  struct mcsafe_ret {

struct mcsafe_ret __mcsafe_copy(void *dst, const void __user *src, size_t cnt);
extern void __mcsafe_copy_end(void);
-#endif

#endif /* __KERNEL__ */
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