Re: [PATCH] KVM: kvmclock: Fix vCPUs > 64 can't be online/hotpluged

[Wanpeng Li]

On Thu, 14 Jan 2021 at 21:45, Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> wrote:
>
> Wanpeng Li <kernellwp@xxxxxxxxx> writes:
>
> > From: Wanpeng Li <wanpengli@xxxxxxxxxxx>
> >
> > 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@xxxxxxxxxxxxxxxxx>
> > Tested-by: Haiwei Li <lihaiwei@xxxxxxxxxxx>
> > Cc: Brijesh Singh <brijesh.singh@xxxxxxx>
> > Cc: stable@xxxxxxxxxxxxxxx#v4.19-rc5+
> > Signed-off-by: Wanpeng Li <wanpengli@xxxxxxxxxxx>
> > ---
> >  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