Re: [RFC] x86, ia32entry: Use sysretl to return from sysenter

[Denys Vlasenko]

On Fri, Mar 27, 2015 at 10:54 PM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
> --- a/arch/x86/ia32/ia32entry.S
> +++ b/arch/x86/ia32/ia32entry.S
> @@ -180,28 +180,34 @@ sysenter_dispatch:
>         testl   $_TIF_ALLWORK_MASK, ASM_THREAD_INFO(TI_flags, %rsp, SIZEOF_PTREGS)
>         jnz     sysexit_audit
>  sysexit_from_sys_call:
> +       /*
> +        * NB: sysexit is not obviously safe for 64-bit kernels -- an
> +        * NMI between sti and sysexit has poorly specified behavior,
> +        * and and NMI followed by an IRQ with usergs is fatal.  So
> +        * we just pretend we're using sysexit but we really use
> +        * sysretl instead.
> +        *
> +        * This code path is still called sysexit because it pairs
> +        * with sysenter and it uses the sysenter calling convention.
> +        */
>         andl    $~TS_COMPAT,ASM_THREAD_INFO(TI_status, %rsp, SIZEOF_PTREGS)
> -       /* clear IF, that popfq doesn't enable interrupts early */
> -       andl    $~0x200,EFLAGS(%rsp)
> -       movl    RIP(%rsp),%edx          /* User %eip */
> -       CFI_REGISTER rip,rdx
> +       movl    RIP(%rsp),%ecx          /* User %eip */
> +       CFI_REGISTER rip,rcx
>         RESTORE_RSI_RDI

I think you need to replace
RESTORE_RSI_RDI with RESTORE_RSI_RDI_RDX

> -       /* pop everything except ss,rsp,rflags slots */
> -       REMOVE_PT_GPREGS_FROM_STACK 3*8
> +       xorl    %edx,%edx

Why do you clear %edx?


>         xorq    %r8,%r8
>         xorq    %r9,%r9
>         xorq    %r10,%r10
> -       xorq    %r11,%r11
> -       popfq_cfi
> +       movl    EFLAGS(%rsp),%r11d      /* User eflags */
>         /*CFI_RESTORE rflags*/
> -       popq_cfi %rcx                           /* User %esp */
> -       CFI_REGISTER rsp,rcx
>         TRACE_IRQS_ON
>         /*
> -        * 32bit SYSEXIT restores eip from edx, esp from ecx.
> -        * cs and ss are loaded from MSRs.
> +        * Sysretl works even on Intel CPUs.  Use it in preference to sysexit,
> +        * since it avoids a dicey window with interrupts enabled.
> +        * CS and SS are loaded from MSRs.

Please do not remove the mini-doc which says what is restored from where.
These instructions are not that obvious. I propose:

         * 64bit->32bit SYSRET restores eip from ecx,
         * eflags from r11 (but RF and VM bits are forced to 0),
         * CS and SS are loaded from MSRs. CS and SS are loaded from MSRs.



Or maybe just

        ...
sysexit_from_sys_call:
        /*
         * Sysretl works even on Intel CPUs.  Use it in preference to sysexit,
         * since it avoids a dicey window with interrupts enabled.
         */
        jmp to sysretl_from_sys_call



and remove the entire "sysexit tail" code path?
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