On Mon, Apr 25, 2022 at 03:41:00PM +0200, Jason A. Donenfeld wrote: > .text:0000000000001A70 add_interrupt_randomness proc near > .text:0000000000001A70 movsxd rcx, edi > .text:0000000000001A73 rdtsc > .text:0000000000001A75 shl rdx, 20h > .text:0000000000001A79 mov rdi, [rsp+0] > .text:0000000000001A7D or rax, rdx > .text:0000000000001A80 mov rdx, offset irq_randomness > .text:0000000000001A87 mov rsi, gs:__irq_regs > .text:0000000000001A8F add rdx, gs:this_cpu_off For context, here's what your suggested change looks like: .text:0000000000001AF0 add_interrupt_randomness proc near .text:0000000000001AF0 push rbx .text:0000000000001AF1 movsxd rbx, edi .text:0000000000001AF4 jmp loc_25AA .text:0000000000001AF9 loc_1AF9: ; CODE XREF: add_interrupt_randomness+AC1↓j .text:0000000000001AF9 rdtsc .text:0000000000001AFB shl rdx, 20h .text:0000000000001AFF or rax, rdx .text:0000000000001B02 .text:0000000000001B02 loc_1B02: ; CODE XREF: add_interrupt_randomness+12C↓j .text:0000000000001B02 mov rdx, offset irq_randomness .text:0000000000001B09 mov rcx, gs:__irq_regs .text:0000000000001B11 add rdx, gs:this_cpu_off [...] .altinstr_aux:00000000000025AA loc_25AA: ; CODE XREF: add_interrupt_randomness+4↑j .altinstr_aux:00000000000025AA test byte ptr cs:boot_cpu_data+20h, 10h .altinstr_aux:00000000000025B1 jnz loc_1AF9 .altinstr_aux:00000000000025B7 jmp loc_1C17 So an extra push, and then that jmp gets nop'd out (I hope). Plus the altinstr area. I guess that's not awful, but I fail to see what we actually gain here over the existing code, which doesn't bother messing with this at runtime when the kernel builder opts out. The ifdef has always been there; what's the reason for adding it now in 2022? Virtualization cannot be it. Jason
