Re: [PATCH 04/13] x86/kvm: Do not try to disable kvmclock if it was not enabled

[Kirill A. Shutemov]

On Fri, Oct 06, 2023 at 07:36:54AM -0700, Sean Christopherson wrote:
> +Paolo
> 
> Please use get_maintainers...

Will do, sorry.

> On Thu, Oct 05, 2023, Kirill A. Shutemov wrote:
> > kvm_guest_cpu_offline() tries to disable kvmclock regardless if it is
> > present in the VM. It leads to write to a MSR that doesn't exist on some
> > configurations, namely in TDX guest:
> > 
> > 	unchecked MSR access error: WRMSR to 0x12 (tried to write 0x0000000000000000)
> > 	at rIP: 0xffffffff8110687c (kvmclock_disable+0x1c/0x30)
> > 
> > kvmclock enabling is gated by CLOCKSOURCE and CLOCKSOURCE2 KVM paravirt
> > features.
> > 
> > Do not disable kvmclock if it was not enumerated or disabled by user
> > from kernel command line.
> > 
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> > Fixes: c02027b5742b ("x86/kvm: Disable kvmclock on all CPUs on shutdown")
> > ---
> >  arch/x86/kernel/kvmclock.c | 9 +++++++--
> >  1 file changed, 7 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/x86/kernel/kvmclock.c b/arch/x86/kernel/kvmclock.c
> > index fb8f52149be9..cba2e732e53f 100644
> > --- a/arch/x86/kernel/kvmclock.c
> > +++ b/arch/x86/kernel/kvmclock.c
> > @@ -22,7 +22,7 @@
> >  #include <asm/x86_init.h>
> >  #include <asm/kvmclock.h>
> >  
> > -static int kvmclock __initdata = 1;
> > +static int kvmclock __ro_after_init = 1;
> >  static int kvmclock_vsyscall __initdata = 1;
> >  static int msr_kvm_system_time __ro_after_init = MSR_KVM_SYSTEM_TIME;
> >  static int msr_kvm_wall_clock __ro_after_init = MSR_KVM_WALL_CLOCK;
> > @@ -195,7 +195,12 @@ static void kvm_setup_secondary_clock(void)
> >  
> >  void kvmclock_disable(void)
> >  {
> > -	native_write_msr(msr_kvm_system_time, 0, 0);
> > +	if (!kvm_para_available() || !kvmclock)
> > +		return;
> > +
> > +	if (kvm_para_has_feature(KVM_FEATURE_CLOCKSOURCE) ||
> > +	    kvm_para_has_feature(KVM_FEATURE_CLOCKSOURCE2))
> > +		native_write_msr(msr_kvm_system_time, 0, 0);
> 
> Rather than recheck everything and preserve kvmclock, what about leaving the MSR
> indices '0' by default and then disable msr_kvm_system_time iff it's non-zero.
> That way the disable path won't become stale if the conditions for enabling
> kvmclock change.

Okay, works for me too.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov