Xin Tong writes: > I am trying to implement a out-of-line TLB lookup for QEMU softmmu-x86-64 on > x86-64 machine, potentially for better instruction cache performance, I have a > few questions.
> 1. I see that tcg_out_qemu_ld_slow_path/tcg_out_qemu_st_slow_path are > generated > when tcg_out_tb_finalize is called. And when a TLB lookup misses, it jumps to > the generated slow path and slow path refills the TLB, then load/store and > jumps > to the next emulated instruction. I am wondering is it easy to outline the > code > for the slow path. I am thinking when a TLB misses, the outlined TLB lookup > code > should generate a call out to the qemu_ld/st_helpers[opc & ~MO_SIGN] and > rewalk > the TLB after its refilled ? This code is off the critical path, so its not as > important as the code when TLB hits. > 2. why not use a TLB or bigger size? currently the TLB has 1<<8 entries. the > TLB > lookup is 10 x86 instructions , but every miss needs ~450 instructions, i > measured this using Intel PIN. so even the miss rate is low (say 3%) the > overall > time spent in the cpu_x86_handle_mmu_fault is still signifcant. I am thinking > the tlb may need to be organized in a set associative fashion to reduce > conflict > miss, e.g. 2 way set associative to reduce the miss rate. or have a victim tlb > that is 4 way associative and use x86 simd instructions to do the lookup once > the direct-mapped tlb misses. Has anybody done any work on this front ? > 3. what are some of the drawbacks of using a superlarge TLB, i.e. a TLB with > 4K > entries ? Using vector intrinsics for the TLB lookup will probably make the code less portable. I don't know how compatible are the GCC and LLVM vectorizing intrinsics between each other (since there has been some efforts on making QEMU also compile with LLVM). A larger TLB will make some operations slower (e.g., look for CPU_TLB_SIZE in cputlb.c), but the higher hit ratio could pay off, although I don't know how the current size was chosen. Lluis -- "And it's much the same thing with knowledge, for whenever you learn something new, the whole world becomes that much richer." -- The Princess of Pure Reason, as told by Norton Juster in The Phantom Tollbooth