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 <l...@amacapital.net> wrote: >On Thu, Dec 13, 2012 at 5:49 PM, H. Peter Anvin <h...@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 -- Sent from my mobile phone. Please excuse brevity and lack of formatting. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/