Commit 9f4ce487 authored Oct 30, 2025 by Yosry Ahmed Committed by Sean Christopherson Nov 13, 2025

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>

parent 4da3768e

Documentation/virt/kvm/x86/errata.rst

+8 −1

Original line number	Diff line number	Diff line
		@@ -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
		------