KVM: x86: Document a virtualization gap for GIF on AMD CPUs

According to the APM Volume #2, Section 15.17, Table 15-10 (24593—Rev.
3.42—March 2024), When "GIF==0", an "Debug exception or trap, due to
breakpoint register match" should be "Ignored and discarded".

KVM lacks any handling of this. Even when vGIF is enabled and vGIF==0,
the CPU does not ignore #DBs and relies on the VMM to do so.

Handling this is possible, but the complexity is unjustified given the
rarity of using HW breakpoints when GIF==0 (e.g. near VMRUN). KVM would
need to intercept the #DB, temporarily disable the breakpoint,
singe-step over the instruction (probably reusing NMI singe-stepping),
and re-enable the breakpoint.

Instead, document this as an erratum.

Signed-off-by: Yosry Ahmed <yosry.ahmed@linux.dev>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Link: https://patch.msgid.link/20251030223757.2950309-1-yosry.ahmed@linux.dev
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: yosrym93

Documentation/virt/kvm/x86/errata.rst

@@ -48,7 +48,14 @@ versus "has_error_code", i.e. KVM's ABI follows AMD behavior.
 Nested virtualization features
 ------------------------------
 
-TBD
+On AMD CPUs, when GIF is cleared, #DB exceptions or traps due to a breakpoint
+register match are ignored and discarded by the CPU. The CPU relies on the VMM
+to fully virtualize this behavior, even when vGIF is enabled for the guest
+(i.e. vGIF=0 does not cause the CPU to drop #DBs when the guest is running).
+KVM does not virtualize this behavior as the complexity is unjustified given
+the rarity of the use case. One way to handle this would be for KVM to
+intercept the #DB, temporarily disable the breakpoint, single-step over the
+instruction, then re-enable the breakpoint.
 
 x2APIC
 ------