KVM: VMX: Allow guest to set DEBUGCTL.RTM_DEBUG if RTM is supported

KVM: VMX: Allow guest to set DEBUGCTL.RTM_DEBUG if RTM is supported

[ Upstream commit 17ec2f965344ee3fd6620bef7ef68792f4ac3af0 ]

Let the guest set DEBUGCTL.RTM_DEBUG if RTM is supported according to the
guest CPUID model, as debug support is supposed to be available if RTM is
supported, and there are no known downsides to letting the guest debug RTM
aborts.

Note, there are no known bug reports related to RTM_DEBUG, the primary
motivation is to reduce the probability of breaking existing guests when a
future change adds a missing consistency check on vmcs12.GUEST_DEBUGCTL
(KVM currently lets L2 run with whatever hardware supports; whoops).

Note #2, KVM already emulates DR6.RTM, and doesn't restrict access to
DR7.RTM.

Fixes: 83c529151ab0 ("KVM: x86: expose Intel cpu new features (HLE, RTM) to guest")
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20250610232010.162191-5-seanjc@google.com
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>