On Tue, 30 Sep 2025 11:31:15 +0100,
Suzuki K Poulose <suzuki.poulose@arm.com> wrote:
>
> Pausing the VM from a vCPU thread doesn't work today, as it waits indefinitely
> for a signal that never comes. By using the "current_kvm_cpu", enlighten the
> kvm__pause() to skip the current CPU and do it inline. This also brings in a
> restriction that a following kvm__continue() must be called from the same vCPU
> thread.
>
> Cc: Will Deacon <will@kernel.org>
> Cc: Oliver Upton <oliver.upton@linux.dev>
> Link: https://lore.kernel.org/all/20230918104028.GA17744@willie-the-truck/
> Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
> ---
> kvm.c | 35 +++++++++++++++++++++++++++++++----
> 1 file changed, 31 insertions(+), 4 deletions(-)
>
> diff --git a/kvm.c b/kvm.c
> index 07089cf1..cc25ecdb 100644
> --- a/kvm.c
> +++ b/kvm.c
> @@ -59,6 +59,8 @@ const char *kvm_exit_reasons[] = {
>
> static int pause_event;
> static DEFINE_MUTEX(pause_lock);
> +static struct kvm_cpu *pause_req_cpu;
> +
> extern struct kvm_ext kvm_req_ext[];
>
> static char kvm_dir[PATH_MAX];
> @@ -573,9 +575,25 @@ void kvm__reboot(struct kvm *kvm)
>
> void kvm__continue(struct kvm *kvm)
> {
> + /*
> + * We must ensure that the resume request comes from the same context
> + * as the one requested the pause, especially if it was issued from a
> + * vCPU thread.
> + */
> + if (current_kvm_cpu) {
> + if (pause_req_cpu != current_kvm_cpu ||
> + !current_kvm_cpu->paused)
> + die("Trying to resume VM from invalid context");
> + current_kvm_cpu->paused = 0;
> + }
> mutex_unlock(&pause_lock);
> }
>
> +/*
> + * Mark all active CPUs as paused, until kvm__continue() is issued.
> + * NOTE: If this is called from a cpu thread, kvm__continue() must
> + * be called from the same thread.
> + */
> void kvm__pause(struct kvm *kvm)
> {
> int i, paused_vcpus = 0;
> @@ -590,10 +608,17 @@ void kvm__pause(struct kvm *kvm)
> if (pause_event < 0)
> die("Failed creating pause notification event");
> for (i = 0; i < kvm->nrcpus; i++) {
> - if (kvm->cpus[i]->is_running && kvm->cpus[i]->paused == 0)
> - pthread_kill(kvm->cpus[i]->thread, SIGKVMPAUSE);
> - else
> - paused_vcpus++;
> + if (kvm->cpus[i]->is_running && kvm->cpus[i]->paused == 0) {
> + if (current_kvm_cpu != kvm->cpus[i]) {
> + pthread_kill(kvm->cpus[i]->thread, SIGKVMPAUSE);
> + continue;
> + } else if (current_kvm_cpu) {
> + current_kvm_cpu->paused = 1;
> + pause_req_cpu = current_kvm_cpu;
This is also set as we leave the function. Why do we need to set it
twice?
> + /* fall through to update our count */
> + }
> + }
> + paused_vcpus++;
> }
>
> while (paused_vcpus < kvm->nrcpus) {
> @@ -604,6 +629,8 @@ void kvm__pause(struct kvm *kvm)
> paused_vcpus += cur_read;
> }
> close(pause_event);
> + /* Remember the context requesting pause */
> + pause_req_cpu = current_kvm_cpu;
> }
>
> void kvm__notify_paused(void)
Thanks,
M.
--
Without deviation from the norm, progress is not possible.