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SubjectRe: [RFC PATCH 00/35] SEV-ES hypervisor support
On Tue, Sep 15, 2020 at 12:22:05PM -0500, Tom Lendacky wrote:
> On 9/14/20 5:59 PM, Sean Christopherson wrote:
> > Given that we don't yet have publicly available KVM code for TDX, what if I
> > generate and post a list of ioctls() that are denied by either SEV-ES or TDX,
> > organized by the denier(s)? Then for the ioctls() that are denied by one and
> > not the other, we add a brief explanation of why it's denied?
> >
> > If that sounds ok, I'll get the list and the TDX side of things posted
> > tomorrow.
>
> That sounds good.

TDX completely blocks the following ioctl()s:

kvm_vcpu_ioctl_interrupt
kvm_vcpu_ioctl_smi
kvm_vcpu_ioctl_x86_setup_mce
kvm_vcpu_ioctl_x86_set_mce
kvm_vcpu_ioctl_x86_get_debugregs
kvm_vcpu_ioctl_x86_set_debugregs
kvm_vcpu_ioctl_x86_get_xsave
kvm_vcpu_ioctl_x86_set_xsave
kvm_vcpu_ioctl_x86_get_xcrs
kvm_vcpu_ioctl_x86_set_xcrs
kvm_arch_vcpu_ioctl_get_regs
kvm_arch_vcpu_ioctl_set_regs
kvm_arch_vcpu_ioctl_get_sregs
kvm_arch_vcpu_ioctl_set_sregs
kvm_arch_vcpu_ioctl_set_guest_debug
kvm_arch_vcpu_ioctl_get_fpu
kvm_arch_vcpu_ioctl_set_fpu

Looking through the code, I think kvm_arch_vcpu_ioctl_get_mpstate() and
kvm_arch_vcpu_ioctl_set_mpstate() should also be disallowed, we just haven't
actually done so.

There are also two helper functions that are "blocked".
dm_request_for_irq_injection() returns false if guest_state_protected, and
post_kvm_run_save() shoves dummy state.

TDX also selectively blocks/skips portions of other ioctl()s so that the
TDX code itself can yell loudly if e.g. .get_cpl() is invoked. The event
injection restrictions are due to direct injection not being allowed (except
for NMIs); all IRQs have to be routed through APICv (posted interrupts) and
exception injection is completely disallowed.

kvm_vcpu_ioctl_x86_get_vcpu_events:
if (!vcpu->kvm->arch.guest_state_protected)
events->interrupt.shadow = kvm_x86_ops.get_interrupt_shadow(vcpu);

kvm_arch_vcpu_put:
if (vcpu->preempted && !vcpu->kvm->arch.guest_state_protected)
vcpu->arch.preempted_in_kernel = !kvm_x86_ops.get_cpl(vcpu);

kvm_vcpu_ioctl_x86_set_vcpu_events:
u32 allowed_flags = KVM_VCPUEVENT_VALID_NMI_PENDING |
KVM_VCPUEVENT_VALID_SIPI_VECTOR |
KVM_VCPUEVENT_VALID_SHADOW |
KVM_VCPUEVENT_VALID_SMM |
KVM_VCPUEVENT_VALID_PAYLOAD;

if (vcpu->kvm->arch.guest_state_protected)
allowed_flags = KVM_VCPUEVENT_VALID_NMI_PENDING;


kvm_arch_vcpu_ioctl_run:
if (vcpu->kvm->arch.guest_state_protected)
kvm_sync_valid_fields = KVM_SYNC_X86_EVENTS;
else
kvm_sync_valid_fields = KVM_SYNC_X86_VALID_FIELDS;


In addition to the more generic guest_state_protected, we also (obviously
tentatively) have a few other flags to deal with aspects of TDX that I'm
fairly certain don't apply to SEV-ES:

tsc_immutable - KVM doesn't have write access to the TSC offset of the
guest.

eoi_intercept_unsupported - KVM can't intercept EOIs (doesn't have access
to EOI bitmaps) and so can't support level
triggered interrupts, at least not without
extra pain.

readonly_mem_unsupported - Secure EPT (analagous to SNP) requires RWX
permissions for all private/encrypted memory.
S-EPT isn't optional, so we get the joy of
adding this right off the bat...

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