On Wed, Sep 16, 2020 at 03:27:13PM -0500, Tom Lendacky wrote: > On 9/16/20 11:49 AM, Sean Christopherson wrote: > > On Wed, Sep 16, 2020 at 11:38:38AM -0500, Tom Lendacky wrote: > >> > >> > >> On 9/16/20 11:02 AM, Sean Christopherson wrote: > >>> On Wed, Sep 16, 2020 at 10:11:10AM -0500, Tom Lendacky wrote: > >>>> On 9/15/20 3:13 PM, Tom Lendacky wrote: > >>>>> On 9/15/20 11:30 AM, Sean Christopherson wrote: > >>>>>> I don't quite follow the "doesn't mean debugging can't be done in the future". > >>>>>> Does that imply that debugging could be supported for SEV-ES guests, even if > >>>>>> they have an encrypted VMSA? > >>>>> > >>>>> Almost anything can be done with software. It would require a lot of > >>>>> hypervisor and guest code and changes to the GHCB spec, etc. So given > >>>>> that, probably just the check for arch.guest_state_protected is enough for > >>>>> now. I'll just need to be sure none of the debugging paths can be taken > >>>>> before the VMSA is encrypted. > >>>> > >>>> So I don't think there's any guarantee that the KVM_SET_GUEST_DEBUG ioctl > >>>> couldn't be called before the VMSA is encrypted, meaning I can't check the > >>>> arch.guest_state_protected bit for that call. So if we really want to get > >>>> rid of the allow_debug() op, I'd need some other way to indicate that this > >>>> is an SEV-ES / protected state guest. > >>> > >>> Would anything break if KVM "speculatively" set guest_state_protected before > >>> LAUNCH_UPDATE_VMSA? E.g. does KVM need to emulate before LAUNCH_UPDATE_VMSA? > >> > >> Yes, the way the code is set up, the guest state (VMSA) is initialized in > >> the same way it is today (mostly) and that state is encrypted by the > >> LAUNCH_UPDATE_VMSA call. I check the guest_state_protected bit to decide > >> on whether to direct the updates to the real VMSA (before it's encrypted) > >> or the GHCB (that's the get_vmsa() function from patch #5). > > > > Ah, gotcha. Would it work to set guest_state_protected[*] from time zero, > > and move vmsa_encrypted to struct vcpu_svm? I.e. keep vmsa_encrypted, but > > use it only for guiding get_vmsa() and related behavior. > > It is mainly __set_sregs() that needs to know when to allow the register > writes and when not to. During guest initialization, __set_sregs is how > some of the VMSA is initialized by Qemu. Hmm. I assume that also means KVM_SET_REGS and KVM_GET_XCRS are also legal before the VMSA is encrypted? If so, then the current behavior of setting vmsa_encrypted "late" make sense. KVM_SET_FPU/XSAVE can be handled by not allocating guest_fpu, i.e. they can be disallowed from time zero without adding an SEV-ES specific check. Which brings us back to KVM_SET_GUEST_DEBUG. What would happen if that were allowed prior to VMSA encryption? If LAUNCH_UPDATE_VMSA acts as a sort of reset, one thought would be to allow KVM_SET_GUEST_DEBUG and then sanitize KVM's state during LAUNCH_UPDATE_VMSA. Or perhaps even better, disallow LAUNCH_UPDATE_VMSA if vcpu->guest_debug!=0. That would allow using debug capabilities up until LAUNCH_UPDATE_VMSA without adding much burden to KVM.
