Re: [Xen-devel] [PATCH v4 4/8] xen/arm: copy dtb fragment to guest dtb

[Julien Grall <julien.grall@xxxxxxx>]

On 24/09/2019 22:06, Stefano Stabellini wrote:
> On Wed, 11 Sep 2019, Julien Grall wrote:
> > On 8/21/19 4:53 AM, Stefano Stabellini wrote:
> > > Signed-off-by: Stefano Stabellini <stefanos@xxxxxxxxxx>
> > >
> > > ----
> > > Changes in v4:
> > > - use recursion in the implementation
> > > - rename handle_properties to handle_prop_pfdt
> > > - rename scan_pt_node to scan_pfdt_node
> > > - pass kinfo to handle_properties
> > > - use uint32_t instead of u32
> > > - rename r to res
> > > - add "passthrough" and "aliases" check
> > > - add a name == NULL check
> > > - code style
> > > - move DTB fragment scanning earlier, before DomU GIC node creation
> > > - set guest_phandle_gic based on "/gic"
> > > - in-code comment
> > >
> > > Changes in v3:
> > > - switch to using device_tree_for_each_node for the copy
> > >
> > > Changes in v2:
> > > - add a note about the code coming from libxl in the commit message
> > > - copy /aliases
> > > - code style
> > > ---
> > >    xen/arch/arm/domain_build.c  | 112 +++++++++++++++++++++++++++++++++++
> > >    xen/include/asm-arm/kernel.h |   2 +-
> > >    2 files changed, 113 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/xen/arch/arm/domain_build.c b/xen/arch/arm/domain_build.c
> > > index cd585f05ca..c71b9f2889 100644
> > > --- a/xen/arch/arm/domain_build.c
> > > +++ b/xen/arch/arm/domain_build.c
> > > @@ -14,6 +14,7 @@
> > >    #include <xen/guest_access.h>
> > >    #include <xen/iocap.h>
> > >    #include <xen/acpi.h>
> > > +#include <xen/vmap.h>
> > >    #include <xen/warning.h>
> > >    #include <acpi/actables.h>
> > >    #include <asm/device.h>
> > > @@ -1713,6 +1714,111 @@ static int __init make_vpl011_uart_node(struct
> > > kernel_info *kinfo)
> > >    }
> > >    #endif
> > >    +static int __init handle_prop_pfdt(struct kernel_info *kinfo,
> > > +                                   const void *pfdt, int nodeoff,
> > > +                                   uint32_t address_cells, uint32_t
> > > size_cells)
> > Why do you need address_cells and size_cells in parameter?
> Yes, it will be necessary for later patches.
ok.

> > > +{
> > > +    void *fdt = kinfo->fdt;
> > > +    int propoff, nameoff, res;
> > > +    const struct fdt_property *prop;
> > > +
> > > +    for ( propoff = fdt_first_property_offset(pfdt, nodeoff);
> > > +          propoff >= 0;
> > > +          propoff = fdt_next_property_offset(pfdt, propoff) )
> > > +    {
> > > +        if ( !(prop = fdt_get_property_by_offset(pfdt, propoff, NULL)) )
> > > +            return -FDT_ERR_INTERNAL;
> > > +
> > > +        nameoff = fdt32_to_cpu(prop->nameoff);
> > > +        res = fdt_property(fdt, fdt_string(pfdt, nameoff),
> > > +                           prop->data, fdt32_to_cpu(prop->len));
> > > +        if ( res )
> > > +            return res;
> > > +    }
> > > +
> > > +    /* FDT_ERR_NOTFOUND => There is no more properties for this node */
> > > +    return ( propoff != -FDT_ERR_NOTFOUND ) ? propoff : 0;
> > > +}
> > > +
> > > +static int __init scan_pfdt_node(struct kernel_info *kinfo, const void
> > > *pfdt,
> > > +                                 int nodeoff, int depth,
> > > +                                 uint32_t address_cells, uint32_t
> > > size_cells)
> > > +{
> > > +    int rc = 0;
> > > +    void *fdt = kinfo->fdt;
> > > +    int node_next;
> > > +    const char *name = fdt_get_name(pfdt, nodeoff, NULL);
> > > +
> > > +    /*
> > > +     * Take the GIC phandle value from the special /gic node in the DTB
> > > +     * fragment.
> > > +     */
> > > +    if ( depth == 1 && dt_node_cmp(name, "gic") == 0 )
> > > +    {
> > > +        kinfo->guest_phandle_gic = fdt_get_phandle(pfdt, nodeoff);
> > > +        return 0;
> > > +    }
> > I don't like this solution. You are bypassing most of the function just for
> > the benefits of have the name in hand. Can this be done separately? This would
> > also avoid to have an extra parameter (depth) for the only benefits of this
> > check.
> All right, I'll change it and remove depth.
Thinking again about this function, you will allow a users to describe a device 
in the node /aliases. So there are more to do in this function.
> > > +
> > > +    rc = fdt_begin_node(fdt, fdt_get_name(pfdt, nodeoff, NULL));
> > > +    if ( rc )
> > > +        return rc;
> > > +
> > > +    rc = handle_prop_pfdt(kinfo, pfdt, nodeoff, address_cells, size_cells);
> > > +    if ( rc )
> > > +        return rc;
> > > +
> > > +    address_cells = device_tree_get_u32(pfdt, nodeoff, "#address-cells",
> > > +                                        address_cells);
> > > +    size_cells = device_tree_get_u32(pfdt, nodeoff, "#size-cells",
> > > +                                     size_cells);
> > I am pretty sure I mention it before (though not on this patch...), this is
> > not matching the DT spec. address_cells and size_cells are not propagated to
> > the next level. So these should be DT_ROOT_NODE_{ADDR, SIZE}_CELLS_DEFAULT.
> They are only propagated from parent to children, not from parent to
> grandchildren. This function is recursive. In this case we are reading
> #address-cells and #size-cells just to pass it on by 1 level as by the
> spec. Am I misunderstanding something?
I am afraid this is not correct. device_tree_get_u32 take the default number of 
cells as 3rd parameter. This is used if the property does not exist.
So if the children does not have the two properties, then you will end up to use 
the parent's value as default when parsing grandchildren "reg" properties.
> > > +
> > > +    node_next = fdt_first_subnode(pfdt, 0);
> > > +    while ( node_next > 0 )
> > > +    {
> > Why do we have to go through the all the nodes of the first level? Can't we
> > just lookup for the path and copy the node as libxl does?
> Yes, we could do that, but fdt_path_offset is implemented as a loop
> anyway and we would still have the same "gic", "aliases" and
> "passthrough" checks. I tried the change but the code doesn't look much
> nicer and we would end up increasing runtime complexity.
This is ok as long as they don't depend on each other. This is not very clear 
from the code that "/gic" does not need to be parsed first, so you may want to 
explain in a comment.
Cheers,

--
Julien Grall

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