Re: [Xen-devel] [PATCH V2 5/5] Xen: switch to linear virtual mapped sparse p2m list

From: David Vrabel
Date: Fri Nov 07 2014 - 09:40:59 EST


On 07/11/14 14:11, Juergen Gross wrote:
> On 11/07/2014 02:54 PM, David Vrabel wrote:
>> On 06/11/14 05:47, Juergen Gross wrote:
>> [...]
>>> @@ -526,23 +411,83 @@ unsigned long get_phys_to_machine(unsigned long
>>> pfn)
>>> return IDENTITY_FRAME(pfn);
>>> }
>>>
>>> - topidx = p2m_top_index(pfn);
>>> - mididx = p2m_mid_index(pfn);
>>> - idx = p2m_index(pfn);
>>> + ptep = lookup_address((unsigned long)(xen_p2m_addr + pfn), &level);
>>> + BUG_ON(!ptep || level != PG_LEVEL_4K);
>>>
>>> /*
>>> * The INVALID_P2M_ENTRY is filled in both p2m_*identity
>>> * and in p2m_*missing, so returning the INVALID_P2M_ENTRY
>>> * would be wrong.
>>> */
>>> - if (p2m_top[topidx][mididx] == p2m_identity)
>>> + if (pte_pfn(*ptep) == PFN_DOWN(__pa(p2m_identity)))
>>> return IDENTITY_FRAME(pfn);
>>>
>>> - return p2m_top[topidx][mididx][idx];
>>> + return xen_p2m_addr[pfn];
>>
>> You should test xen_p2m_addr[pfn] == INVALID_P2M_ENTRY before checking
>> if it's an identity entry. This should skip the more expensive
>> lookup_address() in the common case.
>
> I do. The check is in __pfn_to_mfn(). get_phys_to_machine() is called in
> this case only.

So you do. I missed that.

>>> bool __set_phys_to_machine(unsigned long pfn, unsigned long mfn)
>>
>> I think you should map p2m_missing and p2m_identity as read-only and do
>> the new page allocation on a write fault.
>>
>> set_phys_to_machine() is used every grant map and unmap and in the
>> common case (already allocated array page) it must be a fast and simple:
>>
>> xen_p2m_addr[pfn] = mfn;
>
> Nice idea. I'll try it.

You probably want to try this with a custom exception fixup (or abuse
put_user()).

David

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