On Mon, Mar 21, 2016 at 2:54 PM, Andi Kleen <[email protected]> wrote: > On Mon, Mar 21, 2016 at 02:49:44PM -0400, Brian Gerst wrote: >> On Mon, Mar 21, 2016 at 12:16 PM, Andi Kleen <[email protected]> wrote: >> > From: Andi Kleen <[email protected]> >> > >> > The kernel needs to explicitely enable RD/WRFSBASE to handle context >> > switch correctly. So the application needs to know if it can safely use >> > these instruction. Just looking at the CPUID bit is not enough because it >> > may be running in a kernel that does not enable the instructions. >> > >> > One way for the application would be to just try and catch the SIGILL. >> > But that is difficult to do in libraries which may not want >> > to overwrite the signal handlers of the main application. >> > >> > So we need to provide a way for the application to discover the kernel >> > capability. >> > >> > I used AT_HWCAP2 in the ELF aux vector which is already used by >> > PPC for similar things. We define a new Linux defined bitmap >> > returned in AT_HWCAP. Currently it has only one bit set, >> > for kernel is FSGSBASE capable. >> > >> > The application can then access it manually or using >> > the getauxval() function in newer glibc. >> >> How about adding a VDSO function instead? The VDSO can use >> alternatives, so it can use the new instructions if supported, or else >> use the old syscall. > > What would be the point of that? > > It would be a lot more complicated, and I don't see any advantages > over the aux vector. vdso also requires custom assembler > stubs in the C library. > > -Andi
It would be less complicated actually, as normal userspace would just continue to call arch_prctl() as it does today. Glibc would implement arch_prctl() just like it does with gettimeofday() -- with an ifunc selector [1] that calls the VDSO function if it is available, or the syscall if not. No custom assembly needed. [1] https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/x86/gettimeofday.c;h=36f7c26ffb0e818709d032c605fec8c4bd22a14e;hb=HEAD -- Brian Gerst
