RE: [PATCH 10/11] xen/arm: device assignment on 1:1 direct-map domain

[Penny Zheng <Penny.Zheng@xxxxxxx>]

Hi Julien

> -----Original Message-----
> From: Julien Grall <julien@xxxxxxx>
> Sent: Monday, October 11, 2021 7:14 PM
> To: Penny Zheng <Penny.Zheng@xxxxxxx>; xen-devel@xxxxxxxxxxxxxxxxxxxx;
> sstabellini@xxxxxxxxxx
> Cc: Bertrand Marquis <Bertrand.Marquis@xxxxxxx>; Wei Chen
> <Wei.Chen@xxxxxxx>
> Subject: Re: [PATCH 10/11] xen/arm: device assignment on 1:1 direct-map
> domain
> 
> 
> 
> On 09/10/2021 10:40, Penny Zheng wrote:
> > Hi Julien
> 
> Hi Penny,
> 
> >
> >> -----Original Message-----
> >> From: Julien Grall <julien@xxxxxxx>
> >> Sent: Thursday, September 23, 2021 7:27 PM
> >> To: Penny Zheng <Penny.Zheng@xxxxxxx>;
> >> xen-devel@xxxxxxxxxxxxxxxxxxxx; sstabellini@xxxxxxxxxx
> >> Cc: Bertrand Marquis <Bertrand.Marquis@xxxxxxx>; Wei Chen
> >> <Wei.Chen@xxxxxxx>
> >> Subject: Re: [PATCH 10/11] xen/arm: device assignment on 1:1
> >> direct-map domain
> >>
> >> Hi,
> >>
> >> On 23/09/2021 08:11, Penny Zheng wrote:
> >>> User could do device passthrough, with
> >>> "xen,force-assign-without-iommu" in the device tree snippet, on
> >>> trusted guest through 1:1 direct-map, if IOMMU absent or disabled on
> >> hardware.
> >>
> >> At the moment, it would be possible to passthrough a non-DMA capable
> >> device with direct-mapping. After this patch, this is going to be 
> >> forbidden.
> >>
> >>>
> >>> In order to achieve that, this patch adds 1:1 direct-map check and
> >>> disables iommu-related action.
> >>>
> >>> Signed-off-by: Penny Zheng <penny.zheng@xxxxxxx>
> >>> ---
> >>>    xen/arch/arm/domain_build.c | 12 ++++++++----
> >>>    1 file changed, 8 insertions(+), 4 deletions(-)
> >>>
> >>> diff --git a/xen/arch/arm/domain_build.c
> >>> b/xen/arch/arm/domain_build.c index c92e510ae7..9a9d2522b7 100644
> >>> --- a/xen/arch/arm/domain_build.c
> >>> +++ b/xen/arch/arm/domain_build.c
> >>> @@ -2070,14 +2070,18 @@ static int __init
> >> handle_passthrough_prop(struct kernel_info *kinfo,
> >>>        if ( res < 0 )
> >>>            return res;
> >>>
> >>> +    /*
> >>> +     * If xen_force, we allow assignment of devices without IOMMU
> >> protection.
> >>> +     * And if IOMMU is disabled or absent, 1:1 direct-map is
> >>> + necessary > +
> >> */
> >>> +    if ( xen_force && is_domain_direct_mapped(kinfo->d) &&
> >>> +         !dt_device_is_protected(node) )
> >>
> >> dt_device_is_protected() will be always false unless the device is
> >> protected behing an SMMU using the legacy binding. So I don't think
> >> this is correct to move this check ahead. In fact..
> >>
> >>> +        return 0;
> >>> +
> >>>        res = iommu_add_dt_device(node);
> >>
> >> ... the call should already be a NOP when the IOMMU is disabled or
> >> the device is not behind an IOMMU. So can you explain what you are
> >> trying to prevent here?
> >>
> >
> > If the IOMMU is disabled, iommu_add_dt_device will return 1 as errno.
> > So we could not make it to the xen_force check...
> 
> I disagree. The check is:
> 
> if ( res < 0 )
>    return res;
> 
> Given that res is 1, we wouldn't return and move to check whether the
> assignment can be done.
> 
> >
> > So I tried to move all IOMMU action behind xen_force check.
> >
> > Now, device assignment without IOMMU protection is only applicable on
> > direct-map domains,
> 
> It is fine to assign a non-DMA capable device without direct-mapping. So why
> do you want to add this restriction?
> 

When constructing direct-map-v2, found that, in xen/arch/arm/domain_build.c

if ( dt_find_compatible_node(node, NULL, "multiboot,device-tree") )
    d_cfg.flags |= XEN_DOMCTL_CDF_iommu;

And this flag XEN_DOMCTL_CDF_iommu determines whether iommu is enabled.

In ./xen/include/xen/sched.h

static always_inline bool is_iommu_enabled(const struct domain *d)
{
    return evaluate_nospec(d->options & XEN_DOMCTL_CDF_iommu);
}

That is, even if we assign a non-DMA capable device, we request the platform to 
be
iommu enabled.

> Cheers,
> 
> --
> Julien Grall