Messages in this thread | | | Subject | Re: [PATCH] x86: static_cpu_has_safe: discard dynamic check after init | From | "H. Peter Anvin" <> | Date | Mon, 18 Jan 2016 17:33:03 -0800 |
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On January 18, 2016 5:10:26 PM PST, Borislav Petkov <bp@suse.de> wrote: >On Mon, Jan 18, 2016 at 07:51:07PM +0100, Borislav Petkov wrote: >> [45] .static_cpu_has PROGBITS ffffffff97aa655b 16ea655b >> 00000000000002df 0000000000000000 AX 0 0 1 > >Ok, staring at this section was wrong. I went and looked at >the .s file and now it clicked: gcc inlines those calls to >__static_cpu_has_safe of the dynamic jump target: > > .loc 4 538 0 > movl $125, %edi #, > call __static_cpu_has_safe # >.LBE885: >.LBE886: >.LBE914: > .loc 1 240 0 > testb %al, %al # D.30157 > je .L150 #, > jmp .L151 # >.L152: >.LBB915: >.LBB909: >.LBB905: >.LBB893: >.LBB892: > .loc 4 538 0 > movl $154, %edi #, > call __static_cpu_has_safe # >.LBE892: >.LBE893: >.LBE905: > .loc 7 431 0 > testb %al, %al # D.30157 > jne .L154 #, > >which turn into: > > 751: bf 7d 00 00 00 mov $0x7d,%edi > 756: e8 00 00 00 00 callq 75b <fpu__copy+0xab> > 75b: 84 c0 test %al,%al > 75d: 74 a3 je 702 <fpu__copy+0x52> > 75f: eb 90 jmp 6f1 <fpu__copy+0x41> > 761: bf 9a 00 00 00 mov $0x9a,%edi > 766: e8 00 00 00 00 callq 76b <fpu__copy+0xbb> > 76b: 84 c0 test %al,%al > >That's like 28 bytes in this particular case, which is 14 per call >site, >on average. > >So grepping through my tailored vmlinux, it has 35 entries with >X86_FEATURE_ALWAYS in .altinstructions X 14 = 490 bytes. > >So yeah, we probably should do this, allyesconfig should give more >savings. > >One thing I'd probably do differently is not call the throwaway section >.static_cpu_has but something like .altinstr_temporary or so and put it >after the replacement insns: > >.altinstr_replacement : AT(ADDR(.altinstr_replacement) - LOAD_OFFSET) { > *(.altinstr_replacement) > *(.altinstr_temporary) > } > >so that we know those instructions belong to the alternatives >mechanism. >They'll get discared too, of course. > >I could just as well be talking a lot of crap, it is waay too late >here.
Why the f do we call a subroutine for what amounts to a single bt or test instruction? -- Sent from my Android device with K-9 Mail. Please excuse brevity and formatting.
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