Re: [PATCH 2/4] libxc: support of linear p2m list for migration of pv-domains

[Juergen Gross <jgross@suse.com>]

On 11/12/15 17:09, Andrew Cooper wrote:
> On 11/12/15 16:00, Juergen Gross wrote:
>> On 11/12/15 16:24, Andrew Cooper wrote:
>>>>>> + */
>>>>>> +static int map_p2m_list(struct xc_sr_context *ctx, uint64_t p2m_cr3)
>>>>>> +{
>>>>>> +    xc_interface *xch = ctx->xch;
>>>>>> +    xen_vaddr_t p2m_vaddr, p2m_end, mask, off;
>>>>>> +    xen_pfn_t p2m_mfn, mfn, saved_mfn, max_pfn;
>>>>>> +    uint64_t *ptes;
>>>>>> +    xen_pfn_t *mfns;
>>>>>> +    unsigned fpp, n_pages, level, shift, idx_start, idx_end, idx, saved_idx;
>>>>>> +    int rc = -1;
>>>>>> +
>>>>>> +    p2m_mfn = cr3_to_mfn(ctx, p2m_cr3);
>>>>>> +    if ( p2m_mfn == 0 || p2m_mfn > ctx->x86_pv.max_mfn )
>>>>> mfn 0 isn't invalid to use here.  It could, in principle, be available
>>>>> for PV guest use.
>>>> No, the value 0 indicates that the linear p2m info isn't valid. See
>>>> comments in xen/include/public/arch-x86/xen.h
>>> Technically speaking, that is p2m_cr3, rather than p2m_mfn but I suppose
>>> there is a linear mapping between the two.
>>>
>>> As this function only gets called with a non-zero p2m_cr3, an
>>> alternative would be assert(p2m_cr3 > 0).
>> Hmm, yes.
>>
>>> The mfn == 0 comment also applies for reading the ptes in the loop below.
>> Sure? Is the hypervisor really giving mfn 0 to a guest? I don't mind
>> dropping the test, but I'd be surprised if mfn 0 would be valid.
> 
> Currently no.
> 
> I am thinking longer term for things like a DMLite nested hypervisor,
> where none of the RAM below 1MB is special any more.
> 
> I don't expect handing mfn 0 to guests to actually function very well,
> but I would prefer to avoid false assumptions about it.

Uuh, I really see problems with that approach. A pv guest would have to
check the mfn after allocating the top level page table used to map the
p2m list. Letting mfn 0 be valid is asking for problems, I guess.

I'd rather ban mfn 0 and even gfn 0 from being used as page table, p2m
page or gdt/ldt/idt by pv guests.


Juergen