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

[Vincent Donnefort <vdonnefort@google.com>]

On Sun, Jun 14, 2026 at 01:57:56PM +0100, Fuad Tabba wrote:
> Hi Vincent,
> 
> On Fri, 12 Jun 2026 at 15:22, Vincent Donnefort <vdonnefort@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.

Thanks for the testing!

> 
> 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?

You got me scared for a moment but I did try hVHE and it seems alright:

  [    5.369985] kvm [1]: Protected hVHE mode initialized successfully

  $ ls /sys/kernel/tracing/remotes/hypervisor/

is_kernel_in_hyp_mode() just checks if the kernel is running at EL2 which it
shouldn't in the hVHE case?

> 
> 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
> >