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

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Fri, 20 Sep 2019, Julien Grall wrote:
> 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:
 
The commit message explains what you are doing but I am still missing
something.

Why are we continuing if system_state == SYS_STATE_early_boot and
!is_kernel(va)?

The commit message explains that if system_state == SYS_STATE_early_boot
pagetable pages are static, right? Only after dynamic allocation are
possible it makes sense to use map_domain_page, and dynamic allocations
are possible roughly when system_state switched to SYS_STATE_boot.

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