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From:   Kairui Song <kasong@redhat.com>
Date:   Wed, 27 Feb 2019 22:55:46 +0800
Message-ID: <CACPcB9c86eT=kXcXkBkUjZnifk5bZ71SogfW2tyNudrVmWbnDQ@mail.gmail.com>
Subject: Re: [RFC PATCH] x86, hyperv: fix kernel panic when kexec on HyperV VM
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "K. Y. Srinivasan" <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        Sasha Levin <sashal@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Dave Young <dyoung@redhat.com>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        devel@linuxdriverproject.org
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On Wed, Feb 27, 2019 at 8:02 PM Peter Zijlstra <peterz@infradead.org> wrote:
>
> 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?

Hi, that's for ensuring the hv_hypercall_pg is reset to NULL before
the following wrmsr call. The wrmsr call will make the pointer address
invalid.
I can add some comment in V2 if this is OK.


--
Best Regards,
Kairui Song