On 07/02/2019 12:58, Jan Beulich wrote:
>>>> On 06.02.19 at 21:41, <andrew.coop...@citrix.com> 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.
~Andrew
_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xenproject.org
https://lists.xenproject.org/mailman/listinfo/xen-devel
