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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 17/03/2025 7:56 am, Jan Beulich wrote:
> On 14.03.2025 20:00, Andrew Cooper wrote:
>> On 12/03/2025 9:57 am, Roger Pau Monné wrote:
>>> On Wed, Mar 12, 2025 at 09:31:37AM +0100, Jan Beulich wrote:
>>>> On 11.03.2025 21:47, Andrew Cooper wrote:
>>>>> 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.)
>>>> If in
>>>>
>>>>         .equ    wakeup_stack, trampoline_start + PAGE_SIZE
>>>>
>>>> PAGE_SIZE was replaced by a new (in asm/trampoline.h) TRAMPOLINE_PERM_SIZE,
>>>> you could use PFN_UP(TRAMPOLINE_PERM_SIZE) here to establish a connection.
>>>>
>>>> I have to admit I also don't really see why things going stale here would
>>>> (a) be unlikely and (b) be guaranteed to not go silently.
>> The size can't go to 0 or everything will break, and if it goes larger
>> than 1 (which it almost certainly never will), then APs and/or S3 will
>> break, and we've got both of these in CI.
>>
>> Furthermore, the actual thing which matters is:
>>
>>> /* Map the permanent trampoline page into l1_bootmap[]. */
>>> mov     sym_esi(trampoline_phys), %ecx
>>> lea     __PAGE_HYPERVISOR_RX(%ecx), %edx /* %edx = PTE to write  */
>>> shr     $PAGE_SHIFT, %ecx                /* %ecx = Slot to write */
>>> mov     %edx, sym_offs(l1_bootmap)(%esi, %ecx, 8)
>> which hardcodes 1 page, because there's almost certainly no chance this
>> will ever change.
>>
>>>>  We just don't
>>>> know what we may need to add to the trampoline, sooner or later.
>>> Maybe we could introduce trampoline_{ap?,runtime?}_{start,end} that
>>> covers this more narrow section of the trampoline code?
>> We already have one of those, and a linker assertion that it stays below
>> 1k, so wakeup_stack is at least 3k.
>>
>> The complexity is that the wakeup_stack overlays some init-only logic in
>> the placed trampoline.  It's not something that exists concretely in the
>> Xen image.
> Well - you've got an ack; while I'd prefer if connections were properly
> made, I agree it's unlikely the size will grow enough for it to matter. So
> I think you should feel free to put this in as is.

Thankyou.

~Andrew