Re: [Xen-devel] [[PATCH for-4.13]] xen/arm: mm: Allow generic xen page-tables helpers to be called early

[Julien Grall <julien.grall@xxxxxxx>]

Hi,

On 20/09/2019 00:37, Stefano Stabellini wrote:
> On Tue, 17 Sep 2019, Julien Grall wrote:
> > The current implementations of xen_{map, unmap}_table() expect
> > {map, unmap}_domain_page() to be usable. Those helpers are used to
> > map/unmap page tables while update Xen page-tables.
> >
> > Since commit 022387ee1a "xen/arm: mm: Don't open-code Xen PT update in
> > {set, clear}_fixmap()", setup_fixmap() will make use of the helpers
> > mentioned above. When booting Xen using GRUB, setup_fixmap() may be used
> > before map_domain_page() can be called. This will result to data abort:
> >
> > (XEN) Data Abort Trap. Syndrome=0x5
> > (XEN) CPU0: Unexpected Trap: Data Abort
> >
> > [...]
> >
> > (XEN) Xen call trace:
> > (XEN)    [<000000000025ab6c>] mm.c#xen_pt_update+0x2b4/0x59c (PC)
> > (XEN)    [<000000000025ab20>] mm.c#xen_pt_update+0x268/0x59c (LR)
> > (XEN)    [<000000000025ae70>] set_fixmap+0x1c/0x2c
> > (XEN)    [<00000000002a9c98>] copy_from_paddr+0x7c/0xdc
> > (XEN)    [<00000000002a4ae0>] has_xsm_magic+0x18/0x34
> > (XEN)    [<00000000002a5b5c>] bootfdt.c#early_scan_node+0x398/0x560
> > (XEN)    [<00000000002a5de0>] device_tree_for_each_node+0xbc/0x144
> > (XEN)    [<00000000002a5ed4>] boot_fdt_info+0x6c/0x260
> > (XEN)    [<00000000002ac0d0>] start_xen+0x108/0xc74
> > (XEN)    [<000000000020044c>] arm64/head.o#paging+0x60/0x88
> >
> > During early boot, the page tables are either statically allocated in
> > Xen binary or allocated via alloc_boot_pages().
> >
> > For statically allocated page-tables, they will already be mapped as
> > part of Xen binary. So we can easily find the virtual address.
> >
> > For dynamically allocated page-tables, we need to rely
> > map_domain_page() to be functionally working.
> >
> > For arm32, the call will be usable much before page can be dynamically
> > allocated (see setup_pagetables()). For arm64, the call will be usable
> > after setup_xenheap_mappings().
> >
> > In both cases, memory are given to the boot allocator afterwards. So we
> > can rely on map_domain_page() for mapping page tables allocated
> > dynamically.
> >
> > The helpers xen_{map, unmap}_table() are now updated to take into
> > account the case where page-tables are part of Xen binary.
> >
> > Fixes: 022387ee1a ('xen/arm: mm: Don't open-code Xen PT update in {set, 
> > clear}_fixmap()')
> > Signed-off-by: Julien Grall <julien.grall@xxxxxxx>
> > ---
> >   xen/arch/arm/mm.c | 20 ++++++++++++++++++++
> >   1 file changed, 20 insertions(+)
> >
> > diff --git a/xen/arch/arm/mm.c b/xen/arch/arm/mm.c
> > index e1cdeaaf2f..da6303a8fd 100644
> > --- a/xen/arch/arm/mm.c
> > +++ b/xen/arch/arm/mm.c
> > @@ -950,11 +950,31 @@ static int create_xen_table(lpae_t *entry)
> >   
> >   static lpae_t *xen_map_table(mfn_t mfn)
> >   {
> > +    /*
> > +     * We may require to map the page table before map_domain_page() is
> > +     * useable. The requirements here is it must be useable as soon as
> > +     * page-tables are allocated dynamically via alloc_boot_pages().
> > +     */
> > +    if ( system_state == SYS_STATE_early_boot )
> > +    {
> > +        vaddr_t va = mfn_to_maddr(mfn) - phys_offset;
> > +
> > +        if ( is_kernel(va) )
> > +            return (lpae_t *)va;
> Is it intended to continue if it is not a xen text page? Shouldn't we
> BUG() or WARN?
Yes, I wrote the rationale in the commit message and a summary in a few lines 
above. For convenience, I pasted the commit message again here:
"During early boot, the page tables are either statically allocated in
Xen binary or allocated via alloc_boot_pages().

For statically allocated page-tables, they will already be mapped as
part of Xen binary. So we can easily find the virtual address.

For dynamically allocated page-tables, we need to rely
map_domain_page() to be functionally working.

For arm32, the call will be usable much before page can be dynamically
allocated (see setup_pagetables()). For arm64, the call will be usable
after setup_xenheap_mappings().

In both cases, memory are given to the boot allocator afterwards. So we
can rely on map_domain_page() for mapping page tables allocated
dynamically."

Cheers,

--
Julien Grall

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