Messages in this thread |  | | Date | Mon, 30 Apr 2001 02:13:45 +0200 | From | Andrea Arcangeli <> | Subject | Re: X15 alpha release: as fast as TUX but in user space (fwd) |
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On Sun, Apr 29, 2001 at 04:18:27PM -0400, Gregory Maxwell wrote: > having both the code and a comprehensive jump-table might become tough in a
In the x86-64 implementation there's no jump table. The original design had a jump table but Peter raised the issue that indirect jumps are very costly and he suggested to jump to a fixed virtual address instead, I agreed with his suggestion. So this is what I implemented for x86-64 with regard to the userspace vsyscall API (which will be used by glibc):
enum vsyscall_num { __NR_vgettimeofday, __NR_vtime, }; #define VSYSCALL_ADDR(vsyscall_nr) (VSYSCALL_START+VSYSCALL_SIZE*(vsyscall_nr))
the linker can prelink the vsyscall virtual address into the binary as a weak symbol and the dynamic linker will need to patch it only if somebody is overriding the weak symbol with a LD_PRELOAD.
Virtual address space is relatively cheap. Currently the 64bit vgettimeofday bytecode + data is nearly 200 bytes, and the first two slots are large 512bytes each. So with 1024 bytes we do the whole thing, and we still have space for further 6 vsyscalls without paying any additional tlb entry.
(the implementation of the above #define will change shortly but the VSYSCALL_ADDR() API for glibc will remain the same)
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