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

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Date:	Fri, 7 Nov 2014 14:40:50 +0000
From:	David Vrabel <david.vrabel@...rix.com>
To:	Juergen Gross <jgross@...e.com>, <linux-kernel@...r.kernel.org>,
	<xen-devel@...ts.xensource.com>, <konrad.wilk@...cle.com>,
	<boris.ostrovsky@...cle.com>, <x86@...nel.org>,
	<tglx@...utronix.de>, <mingo@...hat.com>, <hpa@...or.com>
Subject: Re: [Xen-devel] [PATCH V2 5/5] Xen: switch to linear virtual mapped
 sparse p2m list

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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