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

[Andy Lutomirski <luto@amacapital.net>]

On Thu, Dec 13, 2012 at 5:49 PM, H. Peter Anvin <hpa@zytor.com> 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