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

[Kairui Song]

On Tue, Mar 5, 2019 at 8:28 PM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Wed, Feb 27, 2019 at 10:55:46PM +0800, Kairui Song wrote:
> > On Wed, Feb 27, 2019 at 8:02 PM Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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.
>
> WRMSR is a serializing instruction (except for TSC_DEADLINE and the
> X2APIC).
>

Many thanks for the info, I'm not aware of the exception condition, V2
is sent, will drop the barrier in V3 then.

-- 
Best Regards,
Kairui Song