Re: [PATCH v2 2/3] xen/arm: dom0less: Add trap-unmapped-accesses

["Edgar E. Iglesias" <edgar.iglesias@xxxxxxxxx>]

On Thu, May 29, 2025 at 05:03:12PM +0100, Julien Grall wrote:
> Hi Edgar,
> 
> On 29/05/2025 16:50, Edgar E. Iglesias wrote:
> > From: "Edgar E. Iglesias" <edgar.iglesias@xxxxxxx>
> > 
> > Add the trap-unmapped-accesses per-domain fdt property.
> > 
> > Signed-off-by: Edgar E. Iglesias <edgar.iglesias@xxxxxxx>
> > ---
> >   docs/misc/arm/device-tree/booting.txt | 9 +++++++++
> >   xen/arch/arm/dom0less-build.c         | 9 ++++++++-
> >   2 files changed, 17 insertions(+), 1 deletion(-)
> > 
> > diff --git a/docs/misc/arm/device-tree/booting.txt 
> > b/docs/misc/arm/device-tree/booting.txt
> > index 59fa96a82e..8a5c40ddf3 100644
> > --- a/docs/misc/arm/device-tree/booting.txt
> > +++ b/docs/misc/arm/device-tree/booting.txt
> > @@ -225,6 +225,15 @@ with the following properties:
> >       option is provided with a non zero value, but the platform doesn't 
> > support
> >       SVE.
> > +- trap-unmapped-accesses
> > +
> > +    Optional. An integer that configures handling of accesses to unmapped
> > +    address ranges.
> > +    If set to 0, guest accesses will read 0xFFFFFFFF and writes will be 
> > ignored.
> 
> Looking at the code, if I am not mistaken, it will only return this value
> for 32-bit. For 64-bit there will be a few Fs more and for less there will
> be less. So I think this needs to be reworded.
> 
> The rest looks good to me.

Thanks, in v3 I'll change it to:
    If set to 0, guest accesses will read all bits as ones, e.g 0xFFFFFFFF
    for a 32bit access and writes will be ignored.

Cheers,
Edgar

> 
> > +
> > +    This option is only implemented for ARM where the default is 1.
> > +
> >   - xen,enhanced
> >       A string property. Possible property values are:
> > diff --git a/xen/arch/arm/dom0less-build.c b/xen/arch/arm/dom0less-build.c
> > index a4e0a33632..69324aa597 100644
> > --- a/xen/arch/arm/dom0less-build.c
> > +++ b/xen/arch/arm/dom0less-build.c
> > @@ -344,8 +344,15 @@ void __init arch_create_domUs(struct dt_device_node 
> > *node,
> >   #endif
> >       }
> > -    /* Trap accesses to unmapped areas. */
> > +    /* Trap unmapped accesses by default. */
> >       d_cfg->flags |= XEN_DOMCTL_CDF_trap_unmapped_accesses;
> > +    if ( dt_property_read_u32(node, "trap-unmapped-accesses", &val) )
> > +    {
> > +        if ( val > 1 )
> > +            panic("trap-unmapped-accesses: supported values are 0 or 1");
> > +        if ( !val )
> > +            d_cfg->flags &= ~XEN_DOMCTL_CDF_trap_unmapped_accesses;
> > +    }
> >   }
> >   int __init init_intc_phandle(struct kernel_info *kinfo, const char *name,
> 
> Cheers,
> 
> -- 
> Julien Grall
>