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:
> Hi,
> 
> On 20/09/2019 16:16, Stefano Stabellini wrote:
> > 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? 
> That's not correct. Below an excerpt of the commit message:
> 
> "During early boot, the page tables are either statically allocated in
> Xen binary or allocated via alloc_boot_pages()."
> 
> An example of dynamic allocation happening when system_state ==
> SYS_STATE_early_boot is in setup_xenheap_mappings(). alloc_boot_pages() will
> be used to allocate intermediate page-tables as the runtime allocator is not
> yet ready.
> 
> > 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.
> 
> That's not correct. alloc_boot_pages() is actually here to allow dynamic
> allocation before the memory subsystem (and therefore the runtime allocator)
> is initialized.

Let me change the question then: is the system_state ==
SYS_STATE_early_boot check strictly necessary? It looks like it is not:
the patch would work even if it was just:


diff --git a/xen/arch/arm/mm.c b/xen/arch/arm/mm.c
index 9e0fdc39f9..eee7d080c0 100644
--- a/xen/arch/arm/mm.c
+++ b/xen/arch/arm/mm.c
@@ -963,11 +963,19 @@ static int create_xen_table(lpae_t *entry)
 
 static lpae_t *xen_map_table(mfn_t mfn)
 {
+    vaddr_t va = mfn_to_maddr(mfn) - phys_offset;
+
+    if ( is_kernel(va) )
+        return (lpae_t *)va;
+
     return map_domain_page(mfn);
 }
 
 static void xen_unmap_table(const lpae_t *table)
 {
+    if ( is_kernel(table) )
+        return;
+
     unmap_domain_page(table);
 }
 

Is that right? Note that I am not asking you to change the patch, I am
only trying to make sure I am understanding all the implications.

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel