Messages in this thread | | | Date | Thu, 07 May 2015 10:57:49 +0200 | From | Denys Vlasenko <> | Subject | Re: [PATCH] x86: Deinline cpuid_eax and friends |
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On 05/06/2015 10:41 PM, H. Peter Anvin wrote: > On 05/06/2015 12:09 PM, Denys Vlasenko wrote: >>> >>> How on Earth does it make 44 bytes? Is this due to paravirt_fail? >> >> No, just this construct >> >> unsigned int eax, ebx, ecx, edx; >> cpuid(op, &eax, &ebx, &ecx, &edx); >> >> is not really that cheap to set up. You need to allocate >> variables on stack and take address of each: >> >> ffffffff81063668 <cpuid_eax>: >> ffffffff81063668: 55 push %rbp >> ffffffff81063669: 48 89 e5 mov %rsp,%rbp >> ffffffff8106366c: 48 83 ec 10 sub $0x10,%rsp >> ffffffff81063670: 48 8d 4d fc lea -0x4(%rbp),%rcx >> ffffffff81063674: 89 7d f0 mov %edi,-0x10(%rbp) >> ffffffff81063677: 48 8d 55 f8 lea -0x8(%rbp),%rdx >> ffffffff8106367b: 48 8d 75 f4 lea -0xc(%rbp),%rsi >> ffffffff8106367f: 48 8d 7d f0 lea -0x10(%rbp),%rdi >> ffffffff81063683: c7 45 f8 00 00 00 00 movl $0x0,-0x8(%rbp) >> ffffffff8106368a: e8 3c ff ff ff callq ffffffff810635cb <__cpuid> >> ffffffff8106368f: 8b 45 f0 mov -0x10(%rbp),%eax >> ffffffff81063692: c9 leaveq >> ffffffff81063693: c3 retq >> > > That almost certainly is due to paravirt_fail, because otherwise cpuid > would be inline, and gcc actually knows how to optimize around the cpuid > instruction to the point of eliminating the temporaries.
Yes, with HYPERVISOR_GUEST off cpuid_eax() is smaller:
ffffffff81055a66 <cpuid_eax>: ffffffff81055a66: 55 push %rbp ffffffff81055a67: 89 f8 mov %edi,%eax ffffffff81055a69: 31 c9 xor %ecx,%ecx ffffffff81055a6b: 48 89 e5 mov %rsp,%rbp ffffffff81055a6e: 53 push %rbx ffffffff81055a6f: 0f a2 cpuid ffffffff81055a71: 5b pop %rbx ffffffff81055a72: 5d pop %rbp ffffffff81055a73: c3 retq
However, it is not small enough to make vmlinux grow:
text data bss dec hex filename 81746530 13978160 20066304 115790994 6e6d492 vmlinux.before 81746509 13978160 20066304 115790973 6e6d47d vmlinux
To recap: with this patch Code is smaller with and without HYPERVISOR_GUEST. Slowdown per cpuid_REG() call is at worst 4%.
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