On 29.11.2021 18:42, Andrew Cooper wrote:
> On 29/11/2021 17:26, Andrew Cooper wrote:
>> It has been 4 years since the default load address changed from 1M to 2M, and
>> _stext ceased residing in l2_xenmap[0]. We should not be inserting an unused
>> mapping.
>>
>> To ensure we don't create/remove mappings accidentally, loop from 0 and obey
>> _PAGE_PRESENT on all entries.
>>
>> Fixes: 7ed93f3a0dff ("x86: change default load address from 1 MiB to 2 MiB")
>> Signed-off-by: Andrew Cooper <andrew.coop...@citrix.com>
>> ---
>> CC: Jan Beulich <jbeul...@suse.com>
>> CC: Roger Pau Monné <roger....@citrix.com>
>> CC: Wei Liu <w...@xen.org>
>>
>> I ought to have spotted this in c/s 52975142d154 ("x86/boot: Create the
>> l2_xenmap[] mappings dynamically") too.
>> ---
>> xen/arch/x86/setup.c | 10 +++-------
>> 1 file changed, 3 insertions(+), 7 deletions(-)
>>
>> diff --git a/xen/arch/x86/setup.c b/xen/arch/x86/setup.c
>> index da47cdea14a1..6f241048425c 100644
>> --- a/xen/arch/x86/setup.c
>> +++ b/xen/arch/x86/setup.c
>> @@ -1279,16 +1279,12 @@ void __init noreturn __start_xen(unsigned long mbi_p)
>>
>> /* The only data mappings to be relocated are in the Xen area.
>> */
>> pl2e = __va(__pa(l2_xenmap));
>> - /*
>> - * Undo the temporary-hooking of the l1_directmap.
>> __2M_text_start
>> - * is contained in this PTE.
>> - */
>> +
>> BUG_ON(using_2M_mapping() &&
>> l2_table_offset((unsigned long)_erodata) ==
>> l2_table_offset((unsigned long)_stext));
>> - *pl2e++ = l2e_from_pfn(xen_phys_start >> PAGE_SHIFT,
>> - PAGE_HYPERVISOR_RX | _PAGE_PSE);
>> - for ( i = 1; i < L2_PAGETABLE_ENTRIES; i++, pl2e++ )
>> +
>> + for ( i = 0; i < L2_PAGETABLE_ENTRIES; i++, pl2e++ )
>> {
>> unsigned int flags;
>>
>
> Actually, on further consideration, this hunk should be included too, to
> cross-check that we don't zap any of the dynamically created mappings.
I agree in principle, but ...
> --- a/xen/arch/x86/setup.c
> +++ b/xen/arch/x86/setup.c
> @@ -1320,7 +1320,7 @@ void __init noreturn __start_xen(unsigned long mbi_p)
> }
> else
> {
> - *pl2e = l2e_empty();
> + ASSERT_UNREACHABLE();
> continue;
> }
... this isn't going to help non-debug builds. Dropping the zapping would
mean release builds then run with an unadjusted PTE. I can see two options:
Extract the flags from the existing PTE as fallback (i.e. keeping the
ASSERT_UNREACHABLE()) or use BUG() instead.
Jan
