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

[Juergen Gross <jgross@xxxxxxxx>]

On 07/02/2019 14:29, Andrew Cooper wrote:
> On 07/02/2019 12:58, Jan Beulich wrote:
>>>>> 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.
> 
> There is some ancillary logic with vcpu_info_mfn (which looks not to be
> initialised), which is why I suspect this is some more incorrect
> opencoding to begin with.
> 
>>> 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.
> 
> The delta in output (with some of my own debugging) is:
> 
> @@ -22,13 +22,13 @@
>  (XEN)     p2m_base         = 0xffffffffffffffff
>  (XEN)  Xen  kernel: 64-bit, lsb, compat32
>  (XEN)  Dom0 kernel: 32-bit, PAE, lsb, paddr 0x100000 -> 0x112000
> -(XEN) ** nr_pages 241494
> +(XEN) ** nr_pages 241493
>  (XEN) PHYSICAL MEMORY ARRANGEMENT:
> -(XEN)  Dom0 alloc.:   000000003e800000->000000003ec00000 (240470 pages to be 
> allocated) (tot 1024, nr 241494)
> +(XEN)  Dom0 alloc.:   000000003e800000->000000003ec00000 (240469 pages to be 
> allocated) (tot 1024, nr 241493)
>  (XEN) VIRTUAL MEMORY ARRANGEMENT:
>  (XEN)  Loaded kernel: 0000000000100000->0000000000112000
>  (XEN)  Init. ramdisk: 0000000000112000->0000000000112000
> -(XEN)  Phys-Mach map: 0000000000112000->00000000001fdd58
> +(XEN)  Phys-Mach map: 0000000000112000->00000000001fdd54
>  (XEN)  Start info:    00000000001fe000->00000000001fe4b4
>  (XEN)  Xenstore ring: 0000000000000000->0000000000000000
>  (XEN)  Console ring:  0000000000000000->0000000000000000
> 
> I meant the P2M rather than M2P, and it is different by 1 entry which is
> expected, given the change by 1 page.  I've positively identified the
> 1-page change to be the alloc_domheap_page() for the monitor table moving.
> 
>>> @@ -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).
> 
> I did call this out specifically in the commit message.  I had no idea
> about that commit when editing the code, but I still don't understand
> why it is important that the guests top level needs to be first.

In the Linux kernel I have the following comment for 32-bit PV guests:

/*
 * For 32 bit domains xen_start_info->pt_base is the pgd address which
might be
 * not the first page table in the page table pool.
 * Iterate through the initial page tables to find the real page table base.
 */

Does this help somehow?


Juergen

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