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

[Jan Beulich <jbeulich@xxxxxxxx>]

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>
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.

Jan