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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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.

~Andrew

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