Re: [patch] x86/smpboot: Disable parallel bootup if cc_vendor != NONE

Sean Christopherson
2023-05-30T17:32:26Z

On Tue, May 30, 2023, Kirill A. Shutemov wrote:
> On Tue, May 30, 2023 at 06:00:46PM +0200, Thomas Gleixner wrote:
> > On Tue, May 30 2023 at 15:29, Kirill A. Shutemov wrote:
> > > On Tue, May 30, 2023 at 02:09:17PM +0200, Thomas Gleixner wrote:
> > >> The decision to allow parallel bringup of secondary CPUs checks
> > >> CC_ATTR_GUEST_STATE_ENCRYPT to detect encrypted guests. Those cannot use
> > >> parallel bootup because accessing the local APIC is intercepted and raises
> > >> a #VC or #VE, which cannot be handled at that point.
> > >>
> > >> The check works correctly, but only for AMD encrypted guests. TDX does not
> > >> set that flag.
> > >>
> > >> Check for cc_vendor != CC_VENDOR_NONE instead. That might be overbroad, but
> > >> definitely works for both AMD and Intel.
> > >
> > > It boots fine with TDX, but I think it is wrong. cc_get_vendor() will
> > > report CC_VENDOR_AMD even on bare metal if SME is enabled. I don't think
> > > we want it.
> >
> > Right. Did not think about that.
> >
> > But the same way is CC_ATTR_GUEST_MEM_ENCRYPT overbroad for AMD. Only
> > SEV-ES traps RDMSR if I'm understandig that maze correctly.
>
> I don't know difference between SEV flavours that well.
>
> I see there's that on SEV-SNP access to x2APIC MSR range (MSR 0x800-0x8FF)
> is intercepted regardless if MSR_AMD64_SNP_ALT_INJ feature is present. But
> I'm not sure what the state on SEV or SEV-ES.

With SEV-ES, if the hypervisor intercepts an MSR access, the VM-Exit is instead
morphed to a #VC (except for EFER). The guest needs to do an explicit VMGEXIT
(i.e. a hypercall) to explicitly request MSR emulation (this *can* be done in the
#VC handler, but the guest can also do VMGEXIT directly, e.g. in lieu of a RDMSR).

With regular SEV, VM-Exits aren't reflected into the guest. Register state isn't
encrypted so the hypervisor can emulate MSR accesses (and other instructions)
without needing an explicit hypercall from the guest.