On Tue, Jul 29, 2025 at 04:33:53PM +0200, Jan Beulich wrote:
> On 24.07.2025 13:04, Roger Pau Monne wrote:
> > There's a loop in arch_init_memory() that iterates over holes and non-RAM
> > regions to possibly mark any page_info structures matching those addresses
> > as IO. The looping there is done over the PFN space.
> >
> > PFNs not covered by the PDX space will always fail the mfn_valid() check,
> > hence re-write the loop to iterate over the PDX space and avoid checking
> > any holes that are not covered by the PDX translation.
> >
> > On a system with a ~6TiB hole this change together with using PDX
> > compression reduces boot time in approximately 20 seconds. Xen boot time
> > without the change is ~50s, with the change it's ~30s.
>
> That's nice, and I agree what we currently do isn't very efficient, but ...
>
> > --- a/xen/arch/x86/mm.c
> > +++ b/xen/arch/x86/mm.c
> > @@ -275,7 +275,7 @@ static void __init assign_io_page(struct page_info
> > *page)
> >
> > void __init arch_init_memory(void)
> > {
> > - unsigned long i, pfn, rstart_pfn, rend_pfn, iostart_pfn, ioend_pfn;
> > + unsigned long i, pfn, rstart_pfn, rend_pfn, iostart_pfn, ioend_pfn,
> > pdx;
> >
> > /*
> > * Basic guest-accessible flags:
> > @@ -328,9 +328,14 @@ void __init arch_init_memory(void)
> > destroy_xen_mappings((unsigned long)mfn_to_virt(iostart_pfn),
> > (unsigned long)mfn_to_virt(ioend_pfn));
> >
> > - /* Mark as I/O up to next RAM region. */
> > - for ( ; pfn < rstart_pfn; pfn++ )
> > + /*
> > + * Mark as I/O up to next RAM region. Iterate over the PDX space
> > to
> > + * skip holes which would always fail the mfn_valid() check.
> > + */
> > + for ( pdx = pfn_to_pdx(pfn); pdx < pfn_to_pdx(rstart_pfn); pdx++ )
>
> ... pfn_to_pdx() isn't well-defined for a non-RAM PFN, or more precisely for
> any
> PFN that fails the mfn_valid() check. That is, I think, particularly
> noticeable
> with the new offset compression you introduce.
rstart_pfn will always point to the start of the next RAM region (or
the end of the current region if it's the last one). So for that case
pfn_to_pdx() is always provided a RAM PFN as input parameter.
However for the pfn parameter, we would need to do pfn_to_pdx(pfn -
1), as that's the last address in the previous RAM range. The loop
would then possibly be:
for ( pdx = pfn_to_pdx((pfn ?: 1) - 1) + 1; pdx < pfn_to_pdx(rstart_pfn); pdx++
)
{
...
This also assumes that PFN 0 will always have a valid PDX translation,
regardless of whether it's RAM or not (which is the case given the PDX
code currently used).
Thanks, Roger.
