linux-kernel - Re: [PATCH] KVM: kvmclock: Fix vCPUs

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Date:   Fri, 15 Jan 2021 09:15:16 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Brijesh Singh <brijesh.singh@....com>,
        LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>
Subject: Re: [PATCH] KVM: kvmclock: Fix vCPUs > 64 can't be online/hotpluged

On Thu, 14 Jan 2021 at 21:45, Vitaly Kuznetsov <vkuznets@...hat.com> wrote:
>
> Wanpeng Li <kernellwp@...il.com> writes:
>
> > From: Wanpeng Li <wanpengli@...cent.com>
> >
> > The per-cpu vsyscall pvclock data pointer assigns either an element of the
> > static array hv_clock_boot (#vCPU <= 64) or dynamically allocated memory
> > hvclock_mem (vCPU > 64), the dynamically memory will not be allocated if
> > kvmclock vsyscall is disabled, this can result in cpu hotpluged fails in
> > kvmclock_setup_percpu() which returns -ENOMEM. This patch fixes it by not
> > assigning vsyscall pvclock data pointer if kvmclock vdso_clock_mode is not
> > VDSO_CLOCKMODE_PVCLOCK.
> >
> > Fixes: 6a1cac56f4 ("x86/kvm: Use __bss_decrypted attribute in shared variables")
> > Reported-by: Zelin Deng <zelin.deng@...ux.alibaba.com>
> > Tested-by: Haiwei Li <lihaiwei@...cent.com>
> > Cc: Brijesh Singh <brijesh.singh@....com>
> > Cc: stable@...r.kernel.org#v4.19-rc5+
> > Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
> > ---
> >  arch/x86/kernel/kvmclock.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/kernel/kvmclock.c b/arch/x86/kernel/kvmclock.c
> > index aa59374..0624290 100644
> > --- a/arch/x86/kernel/kvmclock.c
> > +++ b/arch/x86/kernel/kvmclock.c
> > @@ -296,7 +296,8 @@ static int kvmclock_setup_percpu(unsigned int cpu)
> >        * pointers. So carefully check. CPU0 has been set up in init
> >        * already.
> >        */
> > -     if (!cpu || (p && p != per_cpu(hv_clock_per_cpu, 0)))
> > +     if (!cpu || (p && p != per_cpu(hv_clock_per_cpu, 0)) ||
> > +         (kvm_clock.vdso_clock_mode != VDSO_CLOCKMODE_PVCLOCK))
> >               return 0;
>
> The comment above should probably be updated as it is not clear why we
> check kvm_clock.vdso_clock_mode here. Actually, I would even suggest we
> introduce a 'kvmclock_tsc_stable' global instead to avoid this indirect
> check.

I prefer to update the comment above, assign vsyscall pvclock data
pointer iff kvmclock vsyscall is enabled.

    Wanpeng