Re: [PATCH 2/2] KVM: LAPIC: Set the TDCR settable bits

[Wanpeng Li]

On Tue, 21 Jul 2020 at 18:51, Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> wrote:
>
> Wanpeng Li <kernellwp@xxxxxxxxx> writes:
>
> > From: Wanpeng Li <wanpengli@xxxxxxxxxxx>
> >
> > Only bits 0, 1, and 3 are settable, others are reserved for APIC_TDCR.
> > Let's record the settable value in the virtual apic page.
> >
> > Signed-off-by: Wanpeng Li <wanpengli@xxxxxxxxxxx>
> > ---
> >  arch/x86/kvm/lapic.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> > index 4ce2ddd..8f7a14d 100644
> > --- a/arch/x86/kvm/lapic.c
> > +++ b/arch/x86/kvm/lapic.c
> > @@ -2068,7 +2068,7 @@ int kvm_lapic_reg_write(struct kvm_lapic *apic, u32 reg, u32 val)
> >       case APIC_TDCR: {
> >               uint32_t old_divisor = apic->divide_count;
> >
> > -             kvm_lapic_set_reg(apic, APIC_TDCR, val);
> > +             kvm_lapic_set_reg(apic, APIC_TDCR, val & 0xb);
> >               update_divide_count(apic);
> >               if (apic->divide_count != old_divisor &&
> >                               apic->lapic_timer.period) {
>
> AFAIU bit 2 should be 0 and other upper bits are reserved. Checking on
> bare hardware,
>
> # wrmsr 0x83e 0xb
> # rdmsr 0x83e
> b
> # wrmsr 0x83e 0xc
> wrmsr: CPU 0 cannot set MSR 0x0000083e to 0x000000000000000c
> # rdmsr 0x83e
> b
>
> Shouldn't we fail the write in case (val & ~0xb) ?

Sorry for the late response since I just come back from vacation. I
can remove the "others are reserved" in patch description for the next
version. It is a little different between Intel and AMD, Intel's bit 2
is 0 and AMD is reserved. On bare-metal, Intel will refuse to set
APIC_TDCR once bits except 0, 1, 3 are setting, however, AMD will
accept bits 0, 1, 3 and ignore other bits setting as patch does.
Before the patch, we can get back anything what we set to the
APIC_TDCR, this patch improves it.

    Wanpeng