Re: [PATCH v2] x86, hyperv: fix kernel panic when kexec on HyperV

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Mar 05, 2019 at 08:17:03PM +0800, Kairui Song wrote:
> diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
> index 7abb09e2eeb8..34aa1e953dfc 100644
> --- a/arch/x86/hyperv/hv_init.c
> +++ b/arch/x86/hyperv/hv_init.c
> @@ -406,6 +406,12 @@ void hyperv_cleanup(void)
>  	/* Reset our OS id */
>  	wrmsrl(HV_X64_MSR_GUEST_OS_ID, 0);
>  
> +	/* Cleanup hypercall page reference before reset the page */
> +	hv_hypercall_pg = NULL;
> +
> +	/* Make sure page reference is cleared before wrmsr */

This comment forgets to tell us who cares about this. And why the wrmsr
itself isn't serializing enough.

> +	wmb();
> +
>  	/* Reset the hypercall page */
>  	hypercall_msr.as_uint64 = 0;
>  	wrmsrl(HV_X64_MSR_HYPERCALL, hypercall_msr.as_uint64);

That looks like a fake MSR; and you're telling me that VMEXIT doesn't
serialize?