Re: [PATCH 05/23] xen/arm: Add capabilities to dom0less

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Sat, 8 Mar 2025, Julien Grall wrote:
> On 08/03/2025 00:02, Jason Andryuk wrote:
> > On 2025-03-07 16:21, Julien Grall wrote:
> > > Hi Jason,
> > > 
> > > On 07/03/2025 17:58, Jason Andryuk wrote:
> > > > On 2025-03-07 04:01, Julien Grall wrote:
> > > > > Hi,
> > > > > 
> > > > > On 06/03/2025 22:03, Jason Andryuk wrote:
> > > > > > Add capabilities property to dom0less to allow building a
> > > > > > disaggregated system.
> > > > > > 
> > > > > > Introduce bootfdt.h to contain these constants.
> > > > > > 
> > > > > > When using the hardware or xenstore capabilities, adjust the grant
> > > > > > and
> > > > > > event channel limits similar to dom0.
> > > > >  > > Also for the hardware domain, set directmap and iommu.  This
> > > > > brings its
> > > > > > configuration in line with a dom0.
> > > > > 
> > > > > Looking the device tree bindings, a user would be allowed to disable
> > > > > "passthrough" or even "directmap". This means, we would never be able
> > > > > to disable "directmap" for the hardware domain in the future with the
> > > > > existing property (this is to avoid break backwards compatibility).
> > > > > 
> > > > > Instead, I think we should check what the user provided and confirm
> > > > > this is matching our expectation for an hardware domain.
> > > >  >
> > > > > That said, I am not entirely sure why we should force directmap for
> > > > > the HW domain. We are starting from a clean slate, so I think it would
> > > > > be better to have by default no directmap and imposing the presence of
> > > > > an IOMMU in the system.
> > > > 
> > > > Ok, it seems like directmap is not necessary.  It was helpful early on
> > > > to get things booting, but I think it's no longer necessary after
> > > > factoring out construct_hwdom().
> > > > 
> > > > What exactly do you mean by imposing with respect to the iommu? Require
> > > > one, or mirror the dom0 code and set it for the hwdom?
> > > > 
> > > >      if ( iommu_enabled )
> > > >          dom0_cfg.flags |= XEN_DOMCTL_CDF_iommu;
> > > 
> > > I mean requires one. Without it, you would need to set directmap and I
> > > don't think this should be allowed (at least for now) for the HW domain.
> > > 
> > > > 
> > > > > Lastly, can you provide an example of what the hardware domain DT node
> > > > > would looke like?
> > > > 
> > > > I've attached a dump of /sys/firmware/fdt from hwdom.  (This is direct
> > > > mapped).
> > > 
> > > Sorry if this was not clear, I am asking for the configuration you wrote
> > > in the host DT for create the hardware domain. I am interested to know
> > > which properties you set...
> > 
> > I've attached the u-boot fdt commands which generate the DT.  Hopefully that
> > works for you.
> > 
> > > > 
> > > > > > --- a/xen/arch/arm/dom0less-build.c
> > > > > > +++ b/xen/arch/arm/dom0less-build.c
> > > > > > @@ -12,6 +12,7 @@
> > > > > >   #include <xen/sizes.h>
> > > > > >   #include <xen/vmap.h>
> > > > > > +#include <public/bootfdt.h>
> > > > > >   #include <public/io/xs_wire.h>
> > > > > >   #include <asm/arm64/sve.h>
> > > > > > @@ -994,6 +995,34 @@ void __init create_domUs(void)
> > > > > >           if ( (max_init_domid + 1) >= DOMID_FIRST_RESERVED )
> > > > > >               panic("No more domain IDs available\n");
> > > > > > +        if ( dt_property_read_u32(node, "capabilities", &val) )
> > > > > > +        {
> > > > > > +            if ( val & ~DOMAIN_CAPS_MASK )
> > > > > > +                panic("Invalid capabilities (%"PRIx32")\n", val);
> > > > > > +
> > > > > > +            if ( val & DOMAIN_CAPS_CONTROL )
> > > > > > +                flags |= CDF_privileged;
> > > > > > +
> > > > > > +            if ( val & DOMAIN_CAPS_HARDWARE )
> > > > > > +            {
> > > > > > +                if ( hardware_domain )
> > > > > > +                    panic("Only 1 hardware domain can be specified!
> > > > > > (%pd)\n",
> > > > > > +                           hardware_domain);
> > > > > > +
> > > > > > +                d_cfg.max_grant_frames = gnttab_dom0_frames();
> > > > > > +                d_cfg.max_evtchn_port = -1;
> > > > > 
> > > > > What about d_cfg.arch.nr_spis? Are we expecting the user to pass
> > > > > "nr_spis"?
> > > > 
> > > > Further down, when nr_spis isn't specified in the DT, it defaults to:
> > > >      d_cfg.arch.nr_spis = gic_number_lines() - 32;
> > > 
> > > Thanks. One further question, what if the user pass "nr_spis" for the HW
> > > domain. Wouldn't this result to more issue further down the line?
> > 
> > I'm not familiar with ARM, so I'll to whatever is best.  I did put the
> > capabilities first, thinking it would set defaults, and then later options
> > could override them.
> 
> I am not sure it is related to Arm. It is more that the HW domain is going to
> re-use the memory layout of the host (this is including the mapping for the
> GIC) and also have all the irqs with pirq == virq.
> 
> I am a bit concerned that letting the users mistakenly tweaking the defaults
> could prevent attaching devices (for instance if the IRQ ID is above >
> nr_spis).
> 
> > 
> > > > 
> > > > Dom0 does:
> > > >      /*
> > > >       * Xen vGIC supports a maximum of 992 interrupt lines.
> > > >       * 32 are substracted to cover local IRQs.
> > > >       */
> > > >      dom0_cfg.arch.nr_spis = min(gic_number_lines(), (unsigned int) 992)
> > > > - 32;
> > > >      if ( gic_number_lines() > 992 )
> > > >          printk(XENLOG_WARNING "Maximum number of vGIC IRQs exceeded.
> > > > \n");
> > > > 
> > > > So I think it's fine as-is?  I could add the min() and warning if you
> > > > think it's necessary.
> > > 
> > > Regardless this discussion, I wonder why we didn't add the min(...) here
> > > like for dom0 because we are using the same vGIC emulation. So the max
> > > SPIs supported is the same...
> > > 
> > > What I am trying to understand is whether it is ok to allow the user to
> > > select "nr_spis", "vpl011" & co if they are either not honored (like for
> > > vpl011) or could introduce further issue (like for nr_spis as the HW
> > > domain is supposed to have the same configuration as dom0).
> > > 
> > > I also don't think it is a good idea to silently ignore what the user
> > > requested. So far, on Arm, we mainly decided to reject/panic early if the
> > > setup is not correct.
> > 
> > Again, I'll do whatever is best.
> 
> Bertrand, Michal, Stefano, any opinions?

As a first step, I would try to make the hardware domain as similar as
dom0 as possible in terms of these configurations. That's because I
would prioritize having something working reliably. So I think it is OK
to panic if the provided configuration is not what we expect, such as
"nr_spis" being provided.

direct-map is different because we know dom0 works fine today without
it, as an example cache coloring requires it. So I think it could be
a good idea to default direct-map to off, also to align better with
the DomU DT bindings. But the address map should still match the host
layout.