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 --- Documentation/virt/kvm/x86/errata.rst | 9 ++++++++- 1 file changed, 8 insertions(+), 1 deletion(-) diff --git a/Documentation/virt/kvm/x86/errata.rst b/Documentation/virt/kvm/x86/errata.rst index 37c79362a48fa..a9cf0e004651a 100644 --- a/Documentation/virt/kvm/x86/errata.rst +++ b/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 ------ -- 2.51.1.930.gacf6e81ea2-goog