Re: [PATCH v2 2/3] x86/viridian: Implement synthetic timer direct mode

["Roger Pau Monné" <roger@xenproject.org>]

On Fri, Sep 04, 2026 at 03:15:15PM +0100, Ross Lagerwall wrote:
> In direct mode, the timer asserts an interrupt on expiration rather than
> using a SynIC message. It is useful to implement this since Windows 11's
> Hyper-V can only use synthetic timers in direct mode.
> 
> Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
> ---
> 
> In v2:
> 
> * Handle migration from older Xen by introducing a new Viridian flag.
> * Added more sanity checks during MSR write and vCPU context load.
> 
>  xen/arch/x86/hvm/viridian/time.c     | 46 ++++++++++++++++++++++++----
>  xen/arch/x86/hvm/viridian/viridian.c |  3 ++
>  xen/include/public/hvm/params.h      |  7 ++++-
>  3 files changed, 49 insertions(+), 7 deletions(-)
> 
> diff --git a/xen/arch/x86/hvm/viridian/time.c b/xen/arch/x86/hvm/viridian/time.c
> index 082528dc9416..4c8352612b19 100644
> --- a/xen/arch/x86/hvm/viridian/time.c
> +++ b/xen/arch/x86/hvm/viridian/time.c
> @@ -223,6 +223,14 @@ static void start_stimer(struct viridian_stimer *vs)
>      set_timer(&vs->timer, timeout + NOW());
>  }
>  
> +static void stimer_deliver_direct(struct vcpu *v, const struct viridian_stimer *vs)
> +{
> +    struct vlapic *vlapic = vcpu_vlapic(v);
> +
> +    if ( vlapic_enabled(vlapic) )
> +        vlapic_set_irq(vlapic, vs->config.apic_vector, 0);
> +}
> +
>  static void poll_stimer(struct vcpu *v, unsigned int stimerx)
>  {
>      struct viridian_vcpu *vv = v->arch.hvm.viridian;
> @@ -242,9 +250,11 @@ static void poll_stimer(struct vcpu *v, unsigned int stimerx)
>      if ( !test_bit(stimerx, &vv->stimer_pending) )
>          return;
>  
> -    if ( !viridian_synic_deliver_timer_msg(v, vs->config.sintx,
> -                                           stimerx, vs->expiration,
> -                                           time_ref_count(v->domain)) )
> +    if ( vs->config.direct_mode )
> +        stimer_deliver_direct(v, vs);
> +    else if ( !viridian_synic_deliver_timer_msg(v, vs->config.sintx,
> +                                                stimerx, vs->expiration,
> +                                                time_ref_count(v->domain)) )
>          return;
>  
>      clear_bit(stimerx, &vv->stimer_pending);
> @@ -361,6 +371,7 @@ int viridian_time_wrmsr(struct vcpu *v, uint32_t idx, uint64_t val)
>      case HV_X64_MSR_STIMER2_CONFIG:
>      case HV_X64_MSR_STIMER3_CONFIG:
>      {
> +        union hv_stimer_config new;
>          unsigned int stimerx = (idx - HV_X64_MSR_STIMER0_CONFIG) / 2;
>          struct viridian_stimer *vs =
>              &array_access_nospec(vv->stimer, stimerx);
> @@ -368,11 +379,18 @@ int viridian_time_wrmsr(struct vcpu *v, uint32_t idx, uint64_t val)
>          if ( !(viridian_feature_mask(d) & HVMPV_stimer) )
>              return X86EMUL_EXCEPTION;
>  
> +        new.as_uint64 = val;
> +        if ( new.direct_mode &&
> +             !(viridian_feature_mask(d) & HVMPV_stimer_direct) )
> +            return X86EMUL_EXCEPTION;

Do we know whether native HyperV also injects a #GP in case of setting
reserved bits on the register?

To keep the previous behavior, should Xen silently ignore the setting
when not supported, like it did in the past?

Thanks, Roger.