Re: [Xen-devel] [PATCH v5 05/10] domain: introduce XEN_DOMCTL_CDF_iommu flag

[Julien Grall <julien.grall@xxxxxxx>]

Hi Paul,

On 14/08/2019 15:26, Paul Durrant wrote:
> > -----Original Message-----
> > From: Julien Grall <julien.grall@xxxxxxx>
> > Sent: 14 August 2019 15:00
> > To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>; xen-devel@xxxxxxxxxxxxxxxxxxxx
> > Cc: Ian Jackson <Ian.Jackson@xxxxxxxxxx>; Wei Liu <wl@xxxxxxx>; Anthony Perard
> > <anthony.perard@xxxxxxxxxx>; Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>; George 
> > Dunlap
> > <George.Dunlap@xxxxxxxxxx>; Jan Beulich <jbeulich@xxxxxxxx>; Konrad Rzeszutek 
> > Wilk
> > <konrad.wilk@xxxxxxxxxx>; Stefano Stabellini <sstabellini@xxxxxxxxxx>; Tim (Xen.org) 
> > <tim@xxxxxxx>;
> > Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>; Roger Pau Monne 
> > <roger.pau@xxxxxxxxxx>
> > Subject: Re: [PATCH v5 05/10] domain: introduce XEN_DOMCTL_CDF_iommu flag
> >
> > Hi Paul,
> >
> > On 14/08/2019 14:38, Paul Durrant wrote:
> > > This patch introduces a common domain creation flag to determine whether
> > > the domain is permitted to make use of the IOMMU. Currently the flag is
> > > always set (for both dom0 and domU) if the IOMMU is globally enabled
> > > (i.e. iommu_enabled == 1). sanitise_domain_config() is modified to reject
> > > the flag if !iommu_enabled.
> > >
> > > A new helper function, is_iommu_enabled(), is added to test the flag and
> > > iommu_domain_init() will return immediately if !is_iommu_enabled(). This is
> > > slightly different to the previous behaviour based on !iommu_enabled where
> > > the call to arch_iommu_domain_init() was made regardless, however it appears
> > > that this call was only necessary to initialize the dt_devices list for ARM
> > > such that iommu_release_dt_devices() can be called unconditionally by
> > > domain_relinquish_resources(). Adding a simple check of is_iommu_enabled()
> > > into iommu_release_dt_devices() keeps this unconditional call working.
> > >
> > > No functional change should be observed with this patch applied.
> > >
> > > Subsequent patches will allow the toolstack to control whether use of the
> > > IOMMU is enabled for a domain.
> > >
> > > NOTE: The introduction of the is_iommu_enabled() helper function might
> > >         seem excessive but its use is expected to increase with subsequent
> > >         patches. Also, having iommu_domain_init() bail before calling
> > >         arch_iommu_domain_init() is not strictly necessary, but I think the
> > >         consequent addition of the call to is_iommu_enabled() in
> > >         iommu_release_dt_devices() makes the code clearer.
> > >
> > > Signed-off-by: Paul Durrant <paul.durrant@xxxxxxxxxx>
> > > ---
> > > Cc: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
> > > Cc: Wei Liu <wl@xxxxxxx>
> > > Cc: Anthony PERARD <anthony.perard@xxxxxxxxxx>
> > > Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> > > Cc: George Dunlap <George.Dunlap@xxxxxxxxxxxxx>
> > > Cc: Jan Beulich <jbeulich@xxxxxxxx>
> > > Cc: Julien Grall <julien.grall@xxxxxxx>
> > > Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> > > Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> > > Cc: Tim Deegan <tim@xxxxxxx>
> > > Cc: Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>
> > > Cc: "Roger Pau Monné" <roger.pau@xxxxxxxxxx>
> > >
> > > Previously part of series 
> > > https://lists.xenproject.org/archives/html/xen-devel/2019-07/msg02267.html
> > >
> > > v5:
> > >    - Move is_iommu_enabled() check into iommu_domain_init()
> > >    - Reject XEN_DOMCTL_CDF_iommu in sanitise_domain_config() if !iommu_enabled
> > >    - Use evaluate_nospec() in defintion of is_iommu_enabled()
> > > ---
> > >    tools/libxl/libxl_create.c            | 8 ++++++++
> > >    xen/arch/arm/domain.c                 | 1 -
> > >    xen/arch/arm/setup.c                  | 3 +++
> > >    xen/arch/x86/setup.c                  | 3 +++
> > >    xen/common/domain.c                   | 9 ++++++++-
> > >    xen/drivers/passthrough/device_tree.c | 3 +++
> > >    xen/drivers/passthrough/iommu.c       | 6 +++---
> > >    xen/include/public/domctl.h           | 4 ++++
> > >    xen/include/xen/sched.h               | 5 +++++
> > >    9 files changed, 37 insertions(+), 5 deletions(-)
> > >
> > > diff --git a/tools/libxl/libxl_create.c b/tools/libxl/libxl_create.c
> > > index 03ce166f4f..050ef042cd 100644
> > > --- a/tools/libxl/libxl_create.c
> > > +++ b/tools/libxl/libxl_create.c
> > > @@ -555,6 +555,7 @@ int libxl__domain_make(libxl__gc *gc, libxl_domain_config 
> > > *d_config,
> > >                .max_grant_frames = b_info->max_grant_frames,
> > >                .max_maptrack_frames = b_info->max_maptrack_frames,
> > >            };
> > > +        libxl_physinfo physinfo;
> > >
> > >            if (info->type != LIBXL_DOMAIN_TYPE_PV) {
> > >                create.flags |= XEN_DOMCTL_CDF_hvm_guest;
> > > @@ -564,6 +565,13 @@ int libxl__domain_make(libxl__gc *gc, libxl_domain_config 
> > > *d_config,
> > >                    libxl_defbool_val(info->oos) ? 0 : XEN_DOMCTL_CDF_oos_off;
> > >            }
> > >
> > > +        rc = libxl_get_physinfo(ctx, &physinfo);
> > > +        if (rc < 0)
> > > +            goto out;
> > > +
> > > +        if (physinfo.cap_hvm_directio)
> > > +            create.flags |= XEN_DOMCTL_CDF_iommu;
> > This is not going to work well on Arm as XEN_SYSCTL_PHYSCAP_directio is never 
> > set.
> Oh, that's true. I need to pull that into the common sysctl handler. It also 
> looks like XEN_SYSCTL_PHYSCAP_hvm should always be set for too, but ARMs 
> arch_do_physinfo() is completely empty at the moment.
Looking at the header (include/public/sysctl.h) it is pretty clear that the two 
PHYSCAP are x86 specific.
Even if Arm guests are based on the same concept as HVM/PVH guest. They are just 
called "Arm guest". We have always taken action so far to say they are HVM/PVH 
guest when presenting to the user. For instance, we strongly recommend to have 
the guest type in your configuration.
So I am not entirely sure we want to expose the capability for Arm. Otherwise a 
user may think it is fine to use "hvm" guest type.
Cheers,

--
Julien Grall

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