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Re: [Xen-devel] [[PATCH for-4.13]] xen/arm: mm: Allow generic xen page-tables helpers to be called early
Hi,
Gentle ping.
Cheers,
On 9/20/19 4:26 PM, 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.
Half of the commit message actually explain when dynamic allocation can
be used. I am not entirely sure what is unclear in it so please suggest
a different commit message.
Cheers,
--
Julien Grall
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