Re: [PATCH] Add VDSO time function support for x86 32-bit kernel

[H. Peter Anvin]

Wouldn't the vdso get mapped already and could be mremap()'d.  If we really need more control I'd almost push for a device/filesystem node that could be mmapped the usual way.

Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:

>On Thu, Dec 13, 2012 at 5:49 PM, H. Peter Anvin <hpa@xxxxxxxxx> wrote:
>> On 12/13/2012 05:42 PM, Andy Lutomirski wrote:
>>>
>>> The 64-bit/x32 case is currently very simple and fast because it
>uses
>>> absolute addressing.  Admittedly, pcrel references are free, so
>>> changing this wouldn't cost much.  For native, it'll be slower, but
>>> maybe no one cares.  I seem to care about this more than anyone
>else,
>>> and I don't use 32 bit code. :)
>>>
>>
>> pcrel is actually cheaper than absolute addressing in 64-bit mode.
>>
>>> The benefit of switching is that the vdso code could be the same in
>>> all three cases.  (Actually, it's even better than that.  All of the
>>> VVAR magic could be the same in the vdso and the kernel -- the
>kernel
>>> linker script would just have to have an appropriate symbol to see
>the
>>> appropriate mapping.)
>>>
>>>
>>> This:
>>>
>>> __attribute__((visibility("hidden"))) int foo;
>>>
>>> int get_foo(void)
>>> {
>>>   return foo;
>>> }
>>>
>>> generates a rip-relative access on 64 bits and GOTOFF on 32 bits.
>>>
>>> The only reason I didn't use a real symbol in the first place is
>>> because I couldn't figure out how to get gcc to emit an absolute
>>> relocation in pic code.
>>
>> Well, then, we wouldn't need to do that... this is starting to sound
>> like a significant win.
>
>How will this avoid breaking checkpoint/restore in userspace?  If the
>vdso is not just plain old code, criu presumably needs to know about
>it.  Should there be an arch_prctl(ARCH_MAP_VDSO, addr) to create a
>vdso mapping somewhere?
>
>--Andy

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