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

[Jan Beulich <jbeulich@xxxxxxxx>]

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.

Jan