Paolo Bonzini [off-list ref] writes:

On 04/10/2017 12:08, Alex Benn?e wrote:

quoted

From 2e8fcea695a9eca67fbeb331d3104d1d9e7e337a Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Alex=20Benn=C3=A9e?= <redacted>
Date: Wed, 4 Oct 2017 09:49:41 +0000
Subject: [PATCH] kvm: exit run loop after emulating IO when single stepping
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If single-stepping is enabled we should exit the run-loop after
emulating the access. Otherwise single-stepping across emulated IO
accesses may skip an instruction.

This only addresses user-space emulation. Stuff done in kernel-mode
should be handled there.

Signed-off-by: Alex Benn?e <redacted>
---
 accel/kvm/kvm-all.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
index 90c88b517d..85bcb2b0d4 100644
--- a/accel/kvm/kvm-all.c
+++ b/accel/kvm/kvm-all.c

@@ -1940,7 +1940,7 @@ int kvm_cpu_exec(CPUState *cpu)
                           run->io.direction,
                           run->io.size,
                           run->io.count);
-            ret = 0;
+            ret = cpu->singlestep_enabled ? EXCP_DEBUG : 0;
             break;
         case KVM_EXIT_MMIO:
             DPRINTF("handle_mmio\n");

@@ -1950,7 +1950,7 @@ int kvm_cpu_exec(CPUState *cpu)
                              run->mmio.data,
                              run->mmio.len,
                              run->mmio.is_write);
-            ret = 0;
+            ret = cpu->singlestep_enabled ? EXCP_DEBUG : 0;
             break;
         case KVM_EXIT_IRQ_WINDOW_OPEN:
             DPRINTF("irq_window_open\n");

Singlestep mode doesn't make much sense for KVM.  For TCG the purpose is
to build one-instruction translation blocks, but what would it mean for KVM?

It's used by the kvm_arch_handle_debug() code to verify single-stepping
is enabled when processing debug exceptions. And also kvm_update_debug:

    if (cpu->singlestep_enabled) {
        data.dbg.control |= KVM_GUESTDBG_ENABLE | KVM_GUESTDBG_SINGLESTEP;
    }

We also have an aliased singlestep_enabled in our disas_context for the
translator.

--
Alex Benn?e