Re: [PATCH kvmtool 2/2] arm64: Add GDB stub and step-debug support

[vince <liuwf0302@gmail.com>]

Hi Ryan,

Thanks, this is a good point.

On arm64, simply patching guest memory with a BRK instruction is not by itself
enough to guarantee that a later guest instruction fetch will observe the new
instruction. This is different from x86, where software breakpoint patching is
generally sufficient without extra instruction-cache maintenance.

I checked QEMU's KVM path as well. While its software breakpoint handling also
patches guest memory directly, I did not find an explicit cache maintenance
sequence in the userspace insertion/removal path.

To make kvmtool's arm64 software breakpoint path more robust, I updated the
implementation so that instruction patches (both inserting the BRK and
restoring the original instruction when stepping over it) go through a
dedicated guest-instruction write path, and arm64 now performs instruction
cache synchronization for those patched bytes.

I have tested this locally and it works as expected.

I'll include this in the next version.

Thanks,
vince