Re: [Xen-devel] help -- A question about ' XENFEAT_auto_translated_physmap':

["Xuquan (Euler)" <xuquan8@xxxxxxxxxx>]

On August 09, 2016 9:02 PM, < JBeulich@xxxxxxxx > wrote:
> >>> On 09.08.16 at 14:36, <xuquan8@xxxxxxxxxx> wrote:
> > Hi Jan,
> >
> > A question about ' XENFEAT_auto_translated_physmap':
> >
> > In linux code, in arch/x86/xen/mmu.c,
>
> I assume you know that I'm not a maintainer of the Linux code.
>
> >  __xen_pgd_walk()
> > {
> > ....
> >
> >         if (xen_feature(XENFEAT_auto_translated_physmap))
> >                 return 0;
> > ....
> > }
> >
> >
> >
> > Why xen_feature(XENFEAT_auto_translated_physmap) is true, then return
> > directly?
> > If not return directly, is there any potential risk?
>
> Well, the function is specifically there for operations (pinning/unpinning) 
> which are required only for the not-auto-translated case.
> Why would anyone want to traverse a page table tree just to do nothing on 
> each of the entries?

Jan, thank you!!
As I am struggling with a dom0 crash, the kernel is old, 3.0.X..
Now there is a crash in 

[<ffffffff80023465>] _pin_lock+0x165/0x2a0    <----*crash*

unable to handle kernel paging request at ffff8b1021826000

static void _pin_lock(struct mm_struct *mm, int lock)
{
171                 pgd_t *pgd = mm->pgd;
172                 unsigned g;
173
174                 for (g = 0; g <= ((TASK_SIZE_MAX-1) / PGDIR_SIZE); g++, 
pgd++) {
175                         pud_t *pud;
176                         unsigned u;
177
178                         if (pgd_none(*pgd))
179                                 continue;
180                         pud = pud_offset(pgd, 0);
181                         for (u = 0; u < PTRS_PER_PUD; u++, pud++) {
182                                 pmd_t *pmd;
183                                 unsigned m;
184
185                                 if (pud_none(*pud))
186                                         continue;
187                                 pmd = pmd_offset(pud, 0);
188                                 for (m = 0; m < PTRS_PER_PMD; m++, pmd++) {
189                                         spinlock_t *ptl;
190
191                                         if (pmd_none(*pmd))          
<---------*crash*
192                                                 continue;
193                                         ptl = pte_lockptr(0, pmd);
194                                         if (lock)
195                                                 spin_lock(ptl);
196                                         else
197                                                 spin_unlock(ptl);
198                                 }
199                         }
200                 }
201         }
202 #endif
}



Quan




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