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

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Date:   Tue, 5 Mar 2019 13:33:29 +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: [PATCH v2] x86, hyperv: fix kernel panic when kexec on HyperV

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?