On Tue, 31 Mar 2015 14:46:21 +0200 Denys Vlasenko <dvlas...@redhat.com> wrote:
> @@ -750,6 +750,53 @@ retint_kernel: > * The iretq could re-enable interrupts: > */ > TRACE_IRQS_IRETQ > + > + /* > + * Since we return to kernel, CS and SS do not need changing. > + * Only RSP, RIP and RFLAGS do. > + * We can use POPF + near RET, which is much faster. > + * The below code may seem excessive, but IRET is _very_ slow. > + * Hundreds of cycles. > + * > + * However, there is a complication. Interrupts in 64-bit mode > + * align stack to 16 bytes. This changes location > + * where we need to store EFLAGS and RIP: > + */ > +#if 0 > + testb $8, RSP(%rsp) Shouldn't this be: testb $0xf, RSP(%rsp) ? The stack should be word (8 bytes) aligned, but I never like to assume anything. As an interrupt can come in anywhere, and %rsp can be modified to anything in assembly, I wouldn't want some "hack" that performs a non word aligned rsp manipulation to suddenly break. It would be rather hard to debug. > + jnz 1f > +#else > + /* There is a complication #2: 64-bit mode has IST stacks */ > + leaq SIZEOF_PTREGS+8(%rsp), %rax > + cmpq %rax, RSP(%rsp) > + je 1f > + subq $8, %rax > + cmpq %rax, RSP(%rsp) > + jne restore_args /* probably IST stack, can't optimize */ > +#endif > + /* there is no padding above iret frame */ > + movq EFLAGS(%rsp), %rax > + movq RIP(%rsp), %rcx > + movq %rax, (SIZEOF_PTREGS-2*8)(%rsp) > + movq %rcx, (SIZEOF_PTREGS-1*8)(%rsp) > + CFI_REMEMBER_STATE > + RESTORE_C_REGS > + REMOVE_PT_GPREGS_FROM_STACK 4*8 /* remove all except last two words */ > + popfq_cfi > + retq BTW, have you made sure that this path has been hit? -- Steve > + CFI_RESTORE_STATE > +1: /* there are 8 bytes of padding above iret frame */ > + movq EFLAGS(%rsp), %rax > + movq RIP(%rsp), %rcx > + movq %rax, (SIZEOF_PTREGS-2*8 + 8)(%rsp) > + movq %rcx, (SIZEOF_PTREGS-1*8 + 8)(%rsp) > + CFI_REMEMBER_STATE > + RESTORE_C_REGS > + REMOVE_PT_GPREGS_FROM_STACK 4*8 + 8 > + popfq_cfi > + retq > + CFI_RESTORE_STATE > + > restore_args: > RESTORE_C_REGS > REMOVE_PT_GPREGS_FROM_STACK 8 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
