RE: [PATCH] x86/hyper-v: Zero out the VP assist page to fix CPU offlining

[Michael Kelley <mikelley@microsoft.com>]

From: Dexuan Cui <decui@microsoft.com> Sent: Wednesday, July 3, 2019 6:46 PM
> 
> When a CPU is being offlined, the CPU usually still receives a few
> interrupts (e.g. reschedule IPIs), after hv_cpu_die() disables the
> HV_X64_MSR_VP_ASSIST_PAGE, so hv_apic_eoi_write() may not write the EOI
> MSR, if the apic_assist field's bit0 happens to be 1; as a result, Hyper-V
> may not be able to deliver all the interrupts to the CPU, and the CPU may
> not be stopped, and the kernel will hang soon.
> 
> The VP ASSIST PAGE is an "overlay" page (see Hyper-V TLFS's Section
> 5.2.1 "GPA Overlay Pages"), so with this fix we're sure the apic_assist
> field is still zero, after the VP ASSIST PAGE is disabled.
> 
> Fixes: ba696429d290 ("x86/hyper-v: Implement EOI assist")
> Signed-off-by: Dexuan Cui <decui@microsoft.com>
> ---
>  arch/x86/hyperv/hv_init.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
> index 0e033ef11a9f..db51a301f759 100644
> --- a/arch/x86/hyperv/hv_init.c
> +++ b/arch/x86/hyperv/hv_init.c
> @@ -60,8 +60,14 @@ static int hv_cpu_init(unsigned int cpu)
>  	if (!hv_vp_assist_page)
>  		return 0;
> 
> +	/*
> +	 * The ZERO flag is necessary, because in the case of CPU offlining
> +	 * the page can still be used by hv_apic_eoi_write() for a while,
> +	 * after the VP ASSIST PAGE is disabled in hv_cpu_die().
> +	 */
>  	if (!*hvp)
> -		*hvp = __vmalloc(PAGE_SIZE, GFP_KERNEL, PAGE_KERNEL);
> +		*hvp = __vmalloc(PAGE_SIZE, GFP_KERNEL | __GFP_ZERO,
> +				 PAGE_KERNEL);
> 
>  	if (*hvp) {
>  		u64 val;
> --
> 2.19.1

Reviewed-by:  Michael Kelley <mikelley@microsoft.com>