Re: [kvm-unit-tests PATCH] x86: Prevent APIC base address from changing in test_enable_x2apic()

[Sean Christopherson <sean.j.christopherson@intel.com>]

On Mon, Apr 15, 2019 at 12:12:58PM -0700, Nadav Amit wrote:
> > On Apr 15, 2019, at 12:09 PM, Nadav Amit <nadav.amit@gmail.com> wrote:
> > 
> >> On Apr 15, 2019, at 12:00 PM, Sean Christopherson <sean.j.christopherson@intel.com> wrote:
> >> 
> >> On Mon, Apr 15, 2019 at 11:31:00AM -0700, nadav.amit@gmail.com wrote:
> >>> From: Nadav Amit <nadav.amit@gmail.com>
> >>> 
> >>> test_enable_x2apic() unintentionally changes the APIC base address to
> >>> zero and resets the BSP indication. This actually causes the local APIC
> >>> to overlap the IDT area, which is wrong. Prevent it from happening by
> >>> keeping the APIC base address and BSP-bit as it was before.
> >>> 
> >>> Signed-off-by: Nadav Amit <nadav.amit@gmail.com>
> >>> ---
> >>> x86/apic.c | 4 ++--
> >>> 1 file changed, 2 insertions(+), 2 deletions(-)
> >>> 
> >>> diff --git a/x86/apic.c b/x86/apic.c
> >>> index 51744cf..0849f87 100644
> >>> --- a/x86/apic.c
> >>> +++ b/x86/apic.c
> >>> @@ -90,11 +90,11 @@ static void test_enable_x2apic(void)
> >>>        report("disabled to x2apic enabled",
> >>>               test_write_apicbase_exception(APIC_EN | APIC_EXTD));
> >>> 
> >>> -        wrmsr(MSR_IA32_APICBASE, APIC_EN);
> >>> +        wrmsr(MSR_IA32_APICBASE, APIC_EN | APIC_DEFAULT_PHYS_BASE | APIC_BSP);
> >>>        report("apic enabled to invalid state",
> >>>               test_write_apicbase_exception(APIC_EXTD));
> >>> 
> >>> -        wrmsr(MSR_IA32_APICBASE, APIC_EN | APIC_EXTD);
> >>> +        wrmsr(MSR_IA32_APICBASE, APIC_EN | APIC_EXTD | APIC_DEFAULT_PHYS_BASE | APIC_BSP);
> >> 
> >> It probably doesn't matter since AFAIK kvm-unit-tests always uses the
> >> default base, but preserving the current base+BSP would be preferred.
> >> The #GP tests get away with using APIC_DEFAULT_PHYS_BASE because the
> >> WRMSR will never succeed, but even that is poor form.  And the test
> >> should also reset to xAPIC when it's done.
> >> 
> >> I think the attached patch covers everything.
> > 
> > Thanks. The base is indeed the default, so it should not matter, but your
> > change should make the code more robust.
> > 
> > One important comment regarding your patch:
> > 
> >> +        if (!(orig_apicbase & APIC_EXTD))
> >> +            reset_apic();
> > 
> > This is not needed, since enable_x2apic() will only return true if it
> > succeeded in enabling x2apic. In addition, this is wrong, since reset_apic()
> > should (usually, and specifically in this case) be followed with:
> > 
> > 	apic_write(APIC_SPIV, 0x1ff);
> > 
> > And not the other way around (there are actually a couple of missing writes
> > to software-enable the APIC after reset_apic(), which I’ll send later).
> 
> Correcting myself - I understand (now) that you want to reset back to xapic, so
> that’s fine. Just keep "apic_write(APIC_SPIV, 0x1ff);” after the reset, please.

Ah, will do, I glazed over the apic_write() and didn't consider its purpose.