Julien Thierry [off-list ref] writes:

On 03/10/17 17:30, Alex Benn?e wrote:

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Julien Thierry [off-list ref] writes:

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On 03/10/17 15:57, Alex Benn?e wrote:

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Julien Thierry [off-list ref] writes:

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On 31/08/17 15:01, Christoffer Dall wrote:

<snip>

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Does that sound like what you had in mind? Or does it seem better than
the
current patch?

I was thinking to change the skip_instruction function to return an
int, and then call kvm_handle_debug_ss() from skip_instruction, which
would update the kvm_run structure and exit here and then.

Setting up the debug exception from within kvm_skip_instruction seem to
change a bit too much its semantic from arm to arm64. I would find this
easily confusing.

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However, I'm now thinking that this doesn't really work either,
because we could have to emulate a trapped MMIO instruction in user
space, and then it's not clear how to exit with a debug exception at
the same time.

A debug exception at guest exit point is (IIRC) just having the
appropriate status in the run->exit_reason (KVM_EXIT_DEBUG). If you need
to exit for MMIO emulation (i.e. the instruction has not run yet) you
shouldn't do that. Exit, emulate and return. We could handle the ioctl
to clear SS in userspace but I guess that gets just as messy.

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So perhaps we should stick with your original approach.

I had not realized that was possible. This makes things more complicated for
avoiding a back and forth with the guest for trapped exceptions. Out of
luck, having the debug flag does look like single stepping would work as
expected for userland MMIOs.

<snip>

This is my currently untested but otherwise simpler solution:

 From 46ea80d7dc9b98661fcd51c41090f8ad74a6690f Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Alex=20Benn=C3=A9e?= <redacted>
Date: Tue, 3 Oct 2017 17:17:15 +0100
Subject: [PATCH] KVM: arm64: handle single-stepping trapped instructions
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

If we are using guest debug to single-step the guest we need to ensure
we exit after emulating the instruction. This only affects
instructions emulated by the kernel. If we exit to userspace anyway we
leave it to userspace to work out what to do.

Signed-off-by: Alex Benn?e <redacted>
---
  arch/arm64/kvm/handle_exit.c | 51 ++++++++++++++++++++++++++++++++------------
  1 file changed, 37 insertions(+), 14 deletions(-)

diff --git a/arch/arm64/kvm/handle_exit.c b/arch/arm64/kvm/handle_exit.c
index 7debb74843a0..b197ffb10e96 100644
--- a/arch/arm64/kvm/handle_exit.c
+++ b/arch/arm64/kvm/handle_exit.c

@@ -178,6 +178,42 @@ static exit_handle_fn kvm_get_exit_handler(struct kvm_vcpu *vcpu)
  	return arm_exit_handlers[hsr_ec];
  }

+/*
+ * When handling traps we need to ensure exit the guest if we
+ * successfully emulated the instruction while single-stepping. If we
+ * have to exit anyway for userspace emulation then it's up to
+ * userspace to handle the "while SSing case".
+ */
+

I have not tested the code but if it work we also need to do something
similar for MMIOs that are handled by the kernel (without returning to
userland). But it should be pretty similar.

<snip>

Which path do they take to the mmio emulation?

--
Alex Benn?e