[PATCH v3 5/6] KVM: x86: implement KVM_GUESTDBG_BLOCKIRQ

Maxim Levitsky
2021-08-11T12:30:59Z

KVM_GUESTDBG_BLOCKIRQ will allow KVM to block all interrupts
while running.

This change is mostly intended for more robust single stepping
of the guest and it has the following benefits when enabled:

* Resuming from a breakpoint is much more reliable.
When resuming execution from a breakpoint, with interrupts enabled,
more often than not, KVM would inject an interrupt and make the CPU
jump immediately to the interrupt handler and eventually return to
the breakpoint, to trigger it again.

From the user point of view it looks like the CPU never executed a
single instruction and in some cases that can even prevent forward
progress, for example, when the breakpoint is placed by an automated
script (e.g lx-symbols), which does something in response to the
breakpoint and then continues the guest automatically.
If the script execution takes enough time for another interrupt to
arrive, the guest will be stuck on the same breakpoint RIP forever.

* Normal single stepping is much more predictable, since it won't
land the debugger into an interrupt handler.

* RFLAGS.TF has less chance to be leaked to the guest:

We set that flag behind the guest's back to do single stepping
but if single step lands us into an interrupt/exception handler
it will be leaked to the guest in the form of being pushed
to the stack.
This doesn't completely eliminate this problem as exceptions
can still happen, but at least this reduces the chances
of this happening.

Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
---
Documentation/virt/kvm/api.rst | 1 +
arch/x86/include/asm/kvm_host.h | 3 ++-
arch/x86/include/uapi/asm/kvm.h | 1 +
arch/x86/kvm/x86.c | 4 ++++
4 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 86d7ad3a126c..4ea1bb28297b 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -3357,6 +3357,7 @@ flags which can include the following:
- KVM_GUESTDBG_INJECT_DB: inject DB type exception [x86]
- KVM_GUESTDBG_INJECT_BP: inject BP type exception [x86]
- KVM_GUESTDBG_EXIT_PENDING: trigger an immediate guest exit [s390]
+ - KVM_GUESTDBG_BLOCKIRQ: avoid injecting interrupts/NMI/SMI [x86]

For example KVM_GUESTDBG_USE_SW_BP indicates that software breakpoints
are enabled in memory so we need to ensure breakpoint exceptions are
diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 72fe03506018..a12db20069d5 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -222,7 +222,8 @@ enum x86_intercept_stage;
KVM_GUESTDBG_USE_HW_BP | \
KVM_GUESTDBG_USE_SW_BP | \
KVM_GUESTDBG_INJECT_BP | \
- KVM_GUESTDBG_INJECT_DB)
+ KVM_GUESTDBG_INJECT_DB | \
+ KVM_GUESTDBG_BLOCKIRQ)

#define PFERR_PRESENT_BIT 0
diff --git a/arch/x86/include/uapi/asm/kvm.h b/arch/x86/include/uapi/asm/kvm.h
index a6c327f8ad9e..2ef1f6513c68 100644
--- a/arch/x86/include/uapi/asm/kvm.h
+++ b/arch/x86/include/uapi/asm/kvm.h
@@ -295,6 +295,7 @@ struct kvm_debug_exit_arch {
#define KVM_GUESTDBG_USE_HW_BP 0x00020000
#define KVM_GUESTDBG_INJECT_DB 0x00040000
#define KVM_GUESTDBG_INJECT_BP 0x00080000
+#define KVM_GUESTDBG_BLOCKIRQ 0x00100000

/* for KVM_SET_GUEST_DEBUG */
struct kvm_guest_debug_arch {
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index e5c7b8fa1f7f..d7fade9e3b94 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -8883,6 +8883,10 @@ static int inject_pending_event(struct kvm_vcpu *vcpu, bool *req_immediate_exit)
can_inject = false;
}

+ /* Don't inject interrupts if the user asked to avoid doing so */
+ if (vcpu->guest_debug & KVM_GUESTDBG_BLOCKIRQ)
+ return 0;
+
/*
* Finally, inject interrupt events. If an event cannot be injected
* due to architectural conditions (e.g. IF=0) a window-open exit
--
2.26.3