When intercepts are enabled for MSR_IA32_XSS, the host will swap in/out
the guest-defined values while context-switching to/from guest mode.
However, in the case of SEV-ES, vcpu->arch.guest_state_protected is set,
so the guest-defined value is effectively ignored when switching to
guest mode with the understanding that the VMSA will handle swapping
in/out this register state.

However, SVM is still configured to intercept these accesses for SEV-ES
guests, so the values in the initial MSR_IA32_XSS are effectively
read-only, and a guest will experience undefined behavior if it actually
tries to write to this MSR. Fortunately, only CET/shadowstack makes use
of this register on SEV-ES-capable systems currently, which isn't yet
widely used, but this may become more of an issue in the future.

Additionally, enabling intercepts of MSR_IA32_XSS results in #VC
exceptions in the guest in certain paths that can lead to unexpected #VC
nesting levels. One example is SEV-SNP guests when handling #VC
exceptions for CPUID instructions involving leaf 0xD, subleaf 0x1, since
they will access MSR_IA32_XSS as part of servicing the CPUID #VC, then
generate another #VC when accessing MSR_IA32_XSS, which can lead to
guest crashes if an NMI occurs at that point in time. Running perf on a
guest while it is issuing such a sequence is one example where these can
be problematic.

Address this by disabling intercepts of MSR_IA32_XSS for SEV-ES guests
if the host/guest configuration allows it. If the host/guest
configuration doesn't allow for MSR_IA32_XSS, leave it intercepted so
that it can be caught by the existing checks in
kvm_{set,get}_msr_common() if the guest still attempts to access it.

Fixes: 376c6d28 ("KVM: SVM: Provide support for SEV-ES vCPU creation/loading")
Cc: Alexey Kardashevskiy <aik@amd.com>
Suggested-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Michael Roth <michael.roth@amd.com>
Message-Id: <20231016132819.1002933-4-michael.roth@amd.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>