Re: [Xen-devel] [PATCH v2 09/12] x86/altp2m: add remaining support routines.

["Lengyel, Tamas" <tlengyel@xxxxxxxxxxx>]

On Thu, Jun 25, 2015 at 12:31 PM, Ed White <edmund.h.white@xxxxxxxxx> wrote:

On 06/24/2015 07:44 PM, Lengyel, Tamas wrote:
>> +    if ( altp2m_active )
>> +    {
>> +        if ( altp2mhvm_hap_nested_page_fault(v, gpa, gla, npfec, &p2m) ==
>> 1 )
>> +        {
>> +            /* entry was lazily copied from host -- retry */
>>
>
> So I'm not fully following this logic here. I can see that the altp2m entry
> got copied from the host. Why is there a need for the retry, why not just
> continue?

At this point the EPT's that the hardware is using have been made valid
by software, but the hardware has already failed the access so you have
to restart the operation. This isn't in any way specific to altp2m,
it's how page fault logic works generally.

Ed

Oh I see, you are working with the assumption that the fault was triggered by the entry not being present in the altp2m EPT, thus it's enough to copy it to resolve the fault. However, if the hostp2m permissions are restricted, there will be a follow-up fault again. Would it maybe make sense to check for that condition and save having to hit two faults?

Tamas

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