Re: [PATCH v16 1/4] xen/arm: Implement PSCI SYSTEM_SUSPEND call for guests

[Mykola Kvach <xakep.amatop@xxxxxxxxx>]

Hi Michal,

Thanks for the review -- I think there are two separate concerns here
(domain state vs. ARM-specific resume context), and it’s easy to conflate
them.

On Thu, Jan 15, 2026 at 11:03 AM Orzel, Michal <michal.orzel@xxxxxxx> wrote:
>
>
>
> On 12/12/2025 14:18, Mykola Kvach wrote:
> > From: Mykola Kvach <mykola_kvach@xxxxxxxx>
> >
> > Add support for the PSCI SYSTEM_SUSPEND function in the vPSCI interface,
> > allowing guests to request suspend via the PSCI v1.0+ SYSTEM_SUSPEND call
> > (both 32-bit and 64-bit variants).
> >
> > Implementation details:
> > - Add SYSTEM_SUSPEND function IDs to PSCI definitions
> > - Trap and handle SYSTEM_SUSPEND in vPSCI
> > - Allow only non-hardware domains to invoke SYSTEM_SUSPEND; return
> >   PSCI_NOT_SUPPORTED for the hardware domain to avoid halting the system
> >   in hwdom_shutdown() via domain_shutdown
> > - Require all secondary VCPUs of the calling domain to be offline before
> >   suspend, as mandated by the PSCI specification
> >
> > The arch_domain_resume() function is an architecture-specific hook that is
> > invoked during domain resume to perform any necessary setup or restoration
> > steps required by the platform. arch_domain_resume() stays int to propagate
> > errno-style detail into common logging; preserving the integer keeps the
> > reason visible and leaves room for future arch-specific failures or richer
> > handling.
> >
> > The new vpsci_vcpu_up_prepare() helper is called on the resume path to set 
> > up
> > the vCPU context (such as entry point, some system regs and context ID) 
> > before
> > resuming a suspended guest. This keeps ARM/vPSCI-specific logic out of 
> > common
> > code and avoids intrusive changes to the generic resume flow.
> >
> > Usage:
> >
> > For Linux-based guests, suspend can be initiated with:
> >     echo mem > /sys/power/state
> > or via:
> >     systemctl suspend
> >
> > Resuming the guest is performed from control domain using:
> >       xl resume <domain>
> >
> > Signed-off-by: Mykola Kvach <mykola_kvach@xxxxxxxx>
> > ---
> > Changes in V16:
> > - Refactor error handling in domain_resume: move logging to generic code,
> >   use explicit return code checking.
> > - Make context clearing conditional on success in arch_domain_resume.
> > - The 'int' return type is retained for arch_domain_resume for consistency
> >   with other arch hooks and to allow for specific negative error codes.
> > ---
> >  xen/arch/arm/domain.c                 |  39 +++++++++
> >  xen/arch/arm/include/asm/domain.h     |   2 +
> >  xen/arch/arm/include/asm/perfc_defn.h |   1 +
> >  xen/arch/arm/include/asm/psci.h       |   2 +
> >  xen/arch/arm/include/asm/suspend.h    |  27 ++++++
> >  xen/arch/arm/include/asm/vpsci.h      |   5 +-
> >  xen/arch/arm/vpsci.c                  | 116 +++++++++++++++++++++-----
> >  xen/common/domain.c                   |  10 +++
> >  xen/include/xen/suspend.h             |  25 ++++++
> >  9 files changed, 205 insertions(+), 22 deletions(-)
> >  create mode 100644 xen/arch/arm/include/asm/suspend.h
> >  create mode 100644 xen/include/xen/suspend.h
> >
> > diff --git a/xen/arch/arm/domain.c b/xen/arch/arm/domain.c
> > index 47973f99d9..f903e7d4f0 100644
> > --- a/xen/arch/arm/domain.c
> > +++ b/xen/arch/arm/domain.c
> > @@ -12,6 +12,8 @@
> >  #include <xen/softirq.h>
> >  #include <xen/wait.h>
> >
> > +#include <public/sched.h>
> > +
> >  #include <asm/arm64/sve.h>
> >  #include <asm/cpuerrata.h>
> >  #include <asm/cpufeature.h>
> > @@ -24,10 +26,12 @@
> >  #include <asm/platform.h>
> >  #include <asm/procinfo.h>
> >  #include <asm/regs.h>
> > +#include <asm/suspend.h>
> >  #include <asm/firmware/sci.h>
> >  #include <asm/tee/tee.h>
> >  #include <asm/vfp.h>
> >  #include <asm/vgic.h>
> > +#include <asm/vpsci.h>
> >  #include <asm/vtimer.h>
> >
> >  #include "vpci.h"
> > @@ -851,6 +855,41 @@ void arch_domain_creation_finished(struct domain *d)
> >      p2m_domain_creation_finished(d);
> >  }
> >
> > +int arch_domain_resume(struct domain *d)
> > +{
> > +    int rc;
> > +    struct resume_info *ctx = &d->arch.resume_ctx;
> > +
> > +    if ( !d->is_shutting_down || d->shutdown_code != SHUTDOWN_suspend )
> How does this check and returning -EINVAL correspond to...

The

  if ( !d->is_shutting_down || d->shutdown_code != SHUTDOWN_suspend )

guard is meant to validate the domain state for the DOMCTL resumedomain
entry point (i.e. reject an xl resume on a domain that isn’t in the
"suspend shutdown" state). Suspend requests issued via SCHEDOP_shutdown /
SCHEDOP_remote_shutdown with reason SUSPEND still put the domain into
is_shutting_down=1 and shutdown_code=SHUTDOWN_suspend, so they do pass
this state check.

What the comment below was trying to say is different: those hypercall
paths don’t go through vPSCI SYSTEM_SUSPEND, so they don’t populate the
Arm-specific resume context (notably wake_cpu). In that case ctx->wake_cpu
remains NULL and the Arm arch_domain_resume() returns early.

>
> > +    {
> > +        dprintk(XENLOG_WARNING,
> > +                "%pd: Invalid domain state for resume: 
> > is_shutting_down=%u, shutdown_code=%u\n",
> > +                d, d->is_shutting_down, d->shutdown_code);
> > +        return -EINVAL;
> > +    }
> > +
> > +    /*
> > +     * It is still possible to call domain_shutdown() with a suspend reason
> > +     * via some hypercalls, such as SCHEDOP_shutdown or 
> > SCHEDOP_remote_shutdown.
> > +     * In these cases, the resume context will be empty.
> this comment? This patch assumes that we can now resume successfully (i.e. 
> this
> function returns 0 and common domain_resume can continue) only if the shutdown
> was with SCHEDOP_shutdown. Anything else will infinitely keep the vCPUS 
> paused.

Separately (and this is why I’m hesitant to make domain_resume()
"suspend-only"): domain_resume() is also used by the soft-reset flow.
Today soft-reset is effectively x86-only (gated by HAS_SOFT_RESET), but
the core plumbing is in common code and is intentionally generic -- the
soft-reset calling chain ends up using domain_resume() as a generic
helper. If domain_resume() itself starts rejecting anything other than
SHUTDOWN_suspend, it would also be a future trap if/when someone
enables HAS_SOFT_RESET on Arm.

>
> Other than that, the patch looks good.
>
> ~Michal
>

Best regards,
Mykola