Re: [PATCH] x86/boot: Fix zap_low_mappings() to map less of the trampoline

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 06/01/2025 11:54 am, Jan Beulich wrote:
> On 06.01.2025 12:26, Andrew Cooper wrote:
>> Regular data access into the trampoline is via the directmap.
>>
>> As now discussed quite extensively in asm/trampoline.h, the trampoline is
>> arranged so that only the AP and S3 paths need an identity mapping, and that
>> they fit within a single page.
>>
>> Right now, PFN_UP(trampoline_end - trampoline_start) is 2, causing more than
>> expected of the trampoline to be mapped.  Cut it down just the single page it
>> ought to be.
>>
>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Acked-by: Jan Beulich <jbeulich@xxxxxxxx>

Thanks.  However,

> on the basis that this improves things. However, ...
>
>> --- a/xen/arch/x86/x86_64/mm.c
>> +++ b/xen/arch/x86/x86_64/mm.c
>> @@ -718,14 +718,16 @@ void __init zap_low_mappings(void)
>>  {
>>      BUG_ON(num_online_cpus() != 1);
>>  
>> -    /* Remove aliased mapping of first 1:1 PML4 entry. */
>> +    /* Stop using l?_bootmap[] mappings. */
>>      l4e_write(&idle_pg_table[0], l4e_empty());
>>      flush_local(FLUSH_TLB_GLOBAL);
>>  
>> -    /* Replace with mapping of the boot trampoline only. */
>> +    /*
>> +     * Insert an identity mapping of the AP/S3 part of the trampoline, which
>> +     * is arranged to fit in a single page.
>> +     */
>>      map_pages_to_xen(trampoline_phys, maddr_to_mfn(trampoline_phys),
>> -                     PFN_UP(trampoline_end - trampoline_start),
>> -                     __PAGE_HYPERVISOR_RX);
>> +                     1, __PAGE_HYPERVISOR_RX);
> ... literal numbers like this - however well they are commented - are
> potentially problematic to locate in case something changes significantly.
> The 1 here really would want connecting with the .equ establishing
> wakeup_stack.

how do you propose doing this?

PFN_UP(wakeup_stack - trampoline_start) doesn't have the same obvious
connection, and it would involve partly undoing 7d73c6f196a5 which hid
the symbol recently.

While 1 isn't ideal, it is next to a comment explaining what's going on,
and it's not going to go stale in a silent way...  (It's also not liable
to go stale either.)

~Andrew