[PATCH v1 0/4] trace_hyp_printk() for pKVM/nVHE hypervisor

[Fuad Tabba]

Hi Vincent,

On Fri, 12 Jun 2026 at 15:22, Vincent Donnefort <vdonnefort at google.com> wrote:
>
> Hi all,
>
> This series adds a hypervisor event "hyp_printk" which enables
> developers to log pretty much anything into the hypervisor tracing
> buffer, just like the kernel function trace_printk().
>
> This enables rich logging from the hypervisor, while leaving all the
> string parsing burden to the kernel. This has been the main way of
> debugging pKVM in Android.

I tested the series on v7.1-rc7 under QEMU (cortex-a53 CPU, pKVM nVHE):
- Booted a host under pKVM with a non-protected kvmtool guest (npVM)
and a protected kvmtool guest (pVM).
- Functional test: added a temporary trace_hyp_printk() call site in
handle___kvm_vcpu_run() with 0-arg, 1-arg, and 2-arg calls. Mounted
tracefs, enabled the hyp_printk event, ran a kvmtool guest to trigger
vcpu_run, read the trace buffer. All expected entries appeared with
correctly formatted output.

One question: kvm_hyp_trace_init() returns early when
is_kernel_in_hyp_mode() is true. On VHE-capable hardware, pKVM uses
hVHE. So it seams that the entire hyp tracing subsystem (not just
hyp_printk) is non-functional in hVHE mode. Is hVHE support
intentionally deferred?

Cheers,
/fuad

>
> Even though not strictly related to trace_hyp_printk, I have added the
> following two patches:
>
>   * KVM: arm64: Allow early calls to pKVM host_share/unshare_hyp
>
>     This one mainly intends to support one of the new features I have
>     posted here [1], which allows to enable tracing as early as
>     possible. I have added it here to limit cross-posting.
>
>   * KVM: arm64: Move kvm_define_hypevents.h to arch/arm64/kvm/
>
>     This one is just a cleanup.
>
> [1] https://lore.kernel.org/all/20260605163825.1762953-1-vdonnefort@google.com/
>
> Vincent Donnefort (4):
>   KVM: arm64: Allow early calls to pKVM host_share/unshare_hyp
>   KVM: arm64: Move kvm_define_hypevents.h to arch/arm64/kvm/
>   tracing/remotes: Add REMOTE_EVENT_CUSTOM_PRINTK() helper
>   KVM: arm64: Add hyp_printk event to nVHE/pKVM hyp
>
>  arch/arm64/include/asm/kvm_asm.h              |  4 +-
>  arch/arm64/include/asm/kvm_hypevents.h        | 14 ++++
>  arch/arm64/include/asm/kvm_hyptrace.h         |  8 +++
>  arch/arm64/kernel/image-vars.h                |  1 +
>  arch/arm64/kernel/vmlinux.lds.S               |  4 ++
>  .../define_hypevents.h}                       |  0
>  .../kvm/hyp/include/nvhe/define_events.h      |  2 -
>  arch/arm64/kvm/hyp/include/nvhe/trace.h       | 65 +++++++++++++++++++
>  arch/arm64/kvm/hyp/nvhe/events.c              |  6 ++
>  arch/arm64/kvm/hyp/nvhe/hyp-main.c            |  2 +-
>  arch/arm64/kvm/hyp_trace.c                    | 60 ++++++++++++++++-
>  include/trace/define_remote_events.h          | 19 +++++-
>  12 files changed, 176 insertions(+), 9 deletions(-)
>  rename arch/arm64/{include/asm/kvm_define_hypevents.h => kvm/define_hypevents.h} (100%)
>
>
> base-commit: 4549871118cf616eecdd2d939f78e3b9e1dddc48
> --
> 2.54.0.1136.gdb2ca164c4-goog
>