Re: [Xen-devel] [PATCH v2] x86/boot: Disable IBRS in intr/nmi exit path at bootup stage

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 27.03.18 at 06:52, <zhenzhong.duan@xxxxxxxxxx> wrote:
> After reset, IBRS is disabled by processor, but a coming intr/nmi leave IBRS
> enabled after their exit. It's not necessory for bootup code to run in low
> performance with IBRS enabled.
> 
> On ORACLE X6-2(500GB/88 cpus, dom0 11GB/20 vcpus), we observed an 200s+ delay
> in construct_dom0.
> 
> By initializing use_shadow_spec_ctrl with the result of (system_state <
> SYS_STATE_active), IBRS is disabled in intr/nmi exit path at bootup stage.
> Then delay in construct_dom0 is ~50s.
> 
> When hot-onlining a CPU, we initialize IBRS early and set use_shadow_spec_ctrl
> to false to avoid Branch Target Injection from sibling threads.
> 
> v2: Use (system_state < SYS_STATE_active) to initialize use_shadow_spec_ctrl
> instead of literal 1 per Jan.

Please place revision information below the first --- marker.

> --- a/xen/include/asm-x86/spec_ctrl.h
> +++ b/xen/include/asm-x86/spec_ctrl.h
> @@ -32,8 +32,22 @@ extern uint8_t default_bti_ist_info;
>  static inline void init_shadow_spec_ctrl_state(void)
>  {
>      struct cpu_info *info = get_cpu_info();
> +    uint32_t val = SPEC_CTRL_IBRS;

Why do you need this variable?

> +    /* Initialize IA32_SPEC_CTRL MSR for hotplugging cpu early */
> +    if ( system_state >= SYS_STATE_active )
> +        asm volatile (ALTERNATIVE(ASM_NOP3, "wrmsr", 
> X86_FEATURE_XEN_IBRS_SET)
> +                      :: "a" (val), "c" (MSR_SPEC_CTRL), "d" (0) : "memory");

I can see the point of doing this, but the title of the patch doesn't
cover it (I think this has been missing independent of your interrupt/
NMI paths consideration).

Further INIT# (unlike RESET#) doesn't clear the register, so you
may want/need to also clear the register in the
X86_FEATURE_XEN_IBRS_CLEAR case.

Also you don't need ASM_NOP3 here after 4008c71d7a ("x86/alt:
Support for automatic padding calculations").

Additionally I think it would be better to keep low and high parts
of the value next to each other in the constraints, rather than
putting the MSR index in the middle.

> -    info->shadow_spec_ctrl = info->use_shadow_spec_ctrl = 0;
> +    info->shadow_spec_ctrl = 0;
> +    /*
> +     * We want to make sure we clear IBRS in interrupt exit path
> +     * (DO_SPEC_CTRL_EXIT_TO_XEN) while dom0 is still booting to
> +     * avoid unnecessary performance impact. As soon as dom0 has
> +     * booted use_shadow_spec_ctrl will be cleared, for example,
> +     * in idle routine.
> +     */
> +    info->use_shadow_spec_ctrl = system_state < SYS_STATE_active;

I think the code overall would be more readable if you had just a
single condition (in if/else form).

And then there is the question of whether to use < / >= or
!= / == : In the resume case, not guest vCPU-s are active (yet),
so perhaps the latter would be better.

In any event please give Andrew a chance to reply before you
send another version, as he may have a different opinion and/or
other valuable input.

Jan


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