linux-kernel - Re: [RFC PATCH] x86, hyperv: fix kernel panic when kexec on HyperV VM

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Message-ID: <20190227120141.GN32494@hirez.programming.kicks-ass.net>
Date:   Wed, 27 Feb 2019 13:01:41 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Kairui Song <kasong@...hat.com>
Cc:     linux-kernel@...r.kernel.org,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Sasha Levin <sashal@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Dave Young <dyoung@...hat.com>, x86@...nel.org,
        devel@...uxdriverproject.org
Subject: Re: [RFC PATCH] x86, hyperv: fix kernel panic when kexec on HyperV VM

On Tue, Feb 26, 2019 at 11:56:15PM +0800, Kairui Song wrote:
>  arch/x86/hyperv/hv_init.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
> index 7abb09e2eeb8..92291c18d716 100644
> --- a/arch/x86/hyperv/hv_init.c
> +++ b/arch/x86/hyperv/hv_init.c
> @@ -406,6 +406,10 @@ void hyperv_cleanup(void)
>  	/* Reset our OS id */
>  	wrmsrl(HV_X64_MSR_GUEST_OS_ID, 0);
>  
> +	/* Cleanup page reference before reset the page */
> +	hv_hypercall_pg = NULL;
> +	wmb();

What do we need that SFENCE for? Any why does it lack a comment?

> +
>  	/* Reset the hypercall page */
>  	hypercall_msr.as_uint64 = 0;
>  	wrmsrl(HV_X64_MSR_HYPERCALL, hypercall_msr.as_uint64);
> -- 
> 2.20.1
>