KVM: x86: Morph pending exceptions to pending VM-Exits at queue time

KVM: x86: Morph pending exceptions to pending VM-Exits at queue time

Morph pending exceptions to pending VM-Exits (due to interception) when
the exception is queued instead of waiting until nested events are
checked at VM-Entry.  This fixes a longstanding bug where KVM fails to
handle an exception that occurs during delivery of a previous exception,
KVM (L0) and L1 both want to intercept the exception (e.g. #PF for shadow
paging), and KVM determines that the exception is in the guest's domain,
i.e. queues the new exception for L2.  Deferring the interception check
causes KVM to esclate various combinations of injected+pending exceptions
to double fault (#DF) without consulting L1's interception desires, and
ends up injecting a spurious #DF into L2.

KVM has fudged around the issue for #PF by special casing emulated #PF
injection for shadow paging, but the underlying issue is not unique to
shadow paging in L0, e.g. if KVM is intercepting #PF because the guest
has a smaller maxphyaddr and L1 (but not L0) is using shadow paging.
Other exceptions are affected as well, e.g. if KVM is intercepting #GP
for one of SVM's workaround or for the VMware backdoor emulation stuff.
The other cases have gone unnoticed because the #DF is spurious if and
only if L1 resolves the exception, e.g. KVM's goofs go unnoticed if L1
would have injected #DF anyways.

The hack-a-fix has also led to ugly code, e.g. bailing from the emulator
if #PF injection forced a nested VM-Exit and the emulator finds itself
back in L1.  Allowing for direct-to-VM-Exit queueing also neatly solves
the async #PF in L2 mess; no need to set a magic flag and token, simply
queue a #PF nested VM-Exit.

Deal with event migration by flagging that a pending exception was queued
by userspace and check for interception at the next KVM_RUN, e.g. so that
KVM does the right thing regardless of the order in which userspace
restores nested state vs. event state.

When "getting" events from userspace, simply drop any pending excpetion
that is destined to be intercepted if there is also an injected exception
to be migrated.  Ideally, KVM would migrate both events, but that would
require new ABI, and practically speaking losing the event is unlikely to
be noticed, let alone fatal.  The injected exception is captured, RIP
still points at the original faulting instruction, etc...  So either the
injection on the target will trigger the same intercepted exception, or
the source of the intercepted exception was transient and/or
non-deterministic, thus dropping it is ok-ish.

Fixes: a04aead144fd ("KVM: nSVM: fix running nested guests when npt=0")
Fixes: feaf0c7dc473 ("KVM: nVMX: Do not generate #DF if #PF happens during exception delivery into L2")
Cc: Jim Mattson <jmattson@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Link: https://lore.kernel.org/r/20220830231614.3580124-22-seanjc@google.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>