Re: [PATCH v3 1/3] x86/ldt: Make modify_ldt synchronous

[Boris Ostrovsky]

On 07/22/2015 06:20 PM, Boris Ostrovsky wrote:
> On 07/22/2015 03:23 PM, Andy Lutomirski wrote:
> > +    error = -ENOMEM;
> > +    new_ldt = alloc_ldt_struct(newsize);
> > +    if (!new_ldt)
> >           goto out_unlock;
> > -    }
> >   -    fill_ldt(&ldt, &ldt_info);
> > -    if (oldmode)
> > -        ldt.avl = 0;
> > +    if (old_ldt) {
> > +        memcpy(new_ldt->entries, old_ldt->entries,
> > +               oldsize * LDT_ENTRY_SIZE);
> > +    }
> > +    memset(new_ldt->entries + oldsize * LDT_ENTRY_SIZE, 0,
> > +           (newsize - oldsize) * LDT_ENTRY_SIZE);
>
> We need to zero out full page (probably better in alloc_ldt_struct() 
> with vmzalloc/__GFP_ZERO) --- Xen checks whole page that is assigned 
> to  G/LDT and gets unhappy if an invalid descriptor is found there.
>
> This fixes one problem. There is something else that Xen gets upset 
> about, I haven't figured what it is yet (and I am out tomorrow so it 
> may need to wait until Friday).


What I thought was another problem turned out not to be one so both 64- 
and 32-bit tests passed on 64-bit PV (when allocated LDT is zeroed out)

However, on 32-bit kernel the test is failing multicpu test, I don't 
know yet what it is.

-boris


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