Re: [Xen-devel] [PATCH] x86/pv: Fix construction of 32bit dom0's

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 06.02.19 at 21:41, <andrew.cooper3@xxxxxxxxxx> wrote:
> Slightly RFC:
> 
> 1) I've not worked out exactly what the
> 
>      v->vcpu_info = (void *)&d->shared_info->compat.vcpu_info[0];
> 
>    line is supposed to be doing and whether it is needed, but it doesn't
>    appear to matter.  It is perhaps another redundant opencoding.

Afaict this is just to be independent of the fact that the vcpu_info
array is first in struct shared_info. I'd be fine with it getting replaced
by a respective BUILD_BUG_ON(), but I'd like to ask that it not be
dropped without replacement.

> 2) The reported
> 
>      Dom0 alloc.:   000000003e800000->000000003ec00000 (240470 pages to be 
> allocated)
> 
>    line changes by 1 page because of the alloc_domheap_page() moving ahead of
>    the printk(), but I'm fairly sure this is benign.  There is a matching
>    reduction in the length of the constructed m2p which is perhaps less
>    benign.

Well, the M2P of course has to be correctly sized. An off-by-one would
likely result in hard to repro bug reports.

> @@ -606,23 +598,14 @@ int __init dom0_construct_pv(struct domain *d,
>      {
>          maddr_to_page(mpt_alloc)->u.inuse.type_info = PGT_l4_page_table;
>          l4start = l4tab = __va(mpt_alloc); mpt_alloc += PAGE_SIZE;
> +        clear_page(l4tab);
> +        init_xen_l4_slots(l4tab, _mfn(virt_to_mfn(l4start)),
> +                          d, INVALID_MFN, true);
> +        v->arch.guest_table = pagetable_from_paddr(__pa(l4start));
>      }
>      else
> -    {
> -        page = alloc_domheap_page(d, MEMF_no_owner | MEMF_no_scrub);
> -        if ( !page )
> -            panic("Not enough RAM for domain 0 PML4\n");
> -        page->u.inuse.type_info = PGT_l4_page_table|PGT_validated|1;
> -        l4start = l4tab = page_to_virt(page);
> -        maddr_to_page(mpt_alloc)->u.inuse.type_info = PGT_l3_page_table;
> -        l3start = __va(mpt_alloc); mpt_alloc += PAGE_SIZE;

This one is lost without replacement, but is needed. Commit
7a9d764630 ("x86/32-on-64: adjust Dom0 initial page table layout")
specifically introduced it to make sure the guest-perceived top level
page table is allocated first (and hence marks the beginning of the
boot page tables, so Dom0 can later put all of them into general use).

Jan


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