* Nadav Amit <nadav.amit@xxxxxxxxx> wrote: > > > On 9/16/14 4:22 PM, Ingo Molnar wrote: > > > > * Nadav Amit <namit@xxxxxxxxxxxxxxxxx> wrote: > > > >> The code that deals with x86 cpuid fields is hard to follow since it performs > >> many bit operations and does not refer to cpuid field explicitly. To > >> eliminate the need of openning a spec whenever dealing with cpuid fields, this > >> patch-set introduces structs that reflect the various cpuid functions. > >> > >> Thanks for reviewing the patch-set. > >> > >> Nadav Amit (3): > >> x86: Adding structs to reflect cpuid fields > >> x86: Use new cpuid structs in cpuid functions > >> KVM: x86: Using cpuid structs in KVM > >> > >> arch/x86/include/asm/cpuid_def.h | 163 +++++++++++++++++++++++++++++++++++++++ > >> arch/x86/kernel/cpu/common.c | 56 ++++++++------ > >> arch/x86/kvm/cpuid.c | 36 +++++---- > >> 3 files changed, 219 insertions(+), 36 deletions(-) > >> create mode 100644 arch/x86/include/asm/cpuid_def.h > > > > I personally like bitfields in theory (they provide type clarity > > and abstract robustness, compared to open-coded bitmask numeric > > literals that are often used in cpuid using code, obfuscating > > cpuid usage), with the big caveat that for many years I didn't > > like bitfields in practice: older versions of GCC did a really > > poor job of optimizing them. > > > > So such a series would only be acceptable if it's demonstrated > > that both 'latest' and 'reasonably old' GCC versions do a good > > job in that department, compared to the old open-coded bitmask > > ops ... > > > > Comparing the 'size vmlinux' output of before/after kernels would > > probably be a good start in seeing the impact of such a change. > > > > If those results are positive then this technique could be > > propagated to all cpuid using code in arch/x86/, of which > > there's plenty. > > Thanks for the quick response. I was not aware GCC behaves this > way. I made some small experiments with GCC-4.8 and GCC-4.4 and > in brief my conclusions are: > > 1. The assembled code of bitmask and bitfields is indeed different. > 2. GCC-4.8 and GCC-4.4 behave pretty much the same, yet GCC-4.8 appears > to make better instructions reordering. > 3. Loading/storing a single bitfield seems to be pretty much optimized > (marginal advantage from code size point-of-view for bitmask, same > number of instructions). > 4. Loading/storing multiple bitfields seems to be somewhat > under-optimized - multiple accesses to the original value result in ~30% > more instructions and code-size. That's better than what I remembered. > So you are correct - bitfields are less optimized. Nonetheless, > since cpuid data is mostly used during startup, and otherwise a > single bitfield is usually accessed in each function - I wonder > whether it worth keeping the optimized but "obfuscate" code. > Obviously, I can guess your answer to this question... So with the condition that you are actively watching out for performance critical code paths, I think the type clarity (i.e. bitfields) is a win. If hpa, tglx or Linus objects I'll yield to that objection though. Opinions, objections? Thanks, Ingo -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
