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

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Date:	Fri, 07 Nov 2014 15:11:18 +0100
From:	Juergen Gross <jgross@...e.com>
To:	David Vrabel <david.vrabel@...rix.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 11/07/2014 02:54 PM, David Vrabel wrote:
> On 06/11/14 05:47, Juergen Gross wrote:
>> At start of the day the Xen hypervisor presents a contiguous mfn list
>> to a pv-domain. In order to support sparse memory this mfn list is
>> accessed via a three level p2m tree built early in the boot process.
>> Whenever the system needs the mfn associated with a pfn this tree is
>> used to find the mfn.
>>
>> Instead of using a software walked tree for accessing a specific mfn
>> list entry this patch is creating a virtual address area for the
>> entire possible mfn list including memory holes. The holes are
>> covered by mapping a pre-defined  page consisting only of "invalid
>> mfn" entries. Access to a mfn entry is possible by just using the
>> virtual base address of the mfn list and the pfn as index into that
>> list. This speeds up the (hot) path of determining the mfn of a
>> pfn.
>>
>> Kernel build on a Dell Latitude E6440 (2 cores, HT) in 64 bit Dom0
>> showed following improvements:
>>
>> Elapsed time: 32:50 ->  32:35
>> System:       18:07 ->  17:47
>> User:        104:00 -> 103:30
>
> After implementing my suggestions below, please provided updated figure.
>   They should be better.

In dom0? I don't think so.

>
>> Tested on 64 bit dom0 and 32 bit domU.
> [...]
>> --- a/arch/x86/include/asm/xen/page.h
>> +++ b/arch/x86/include/asm/xen/page.h
>> @@ -59,6 +59,23 @@ extern int clear_foreign_p2m_mapping(struct gnttab_unmap_grant_ref *unmap_ops,
>>   				     struct page **pages, unsigned int count);
>>   extern unsigned long m2p_find_override_pfn(unsigned long mfn, unsigned long pfn);
>>
>> +static inline unsigned long __pfn_to_mfn(unsigned long pfn)
>
> These variations of pfn_to_mfn() (__pfn_to_mfn() and
> get_phys_to_machine() and any others), need comments explaining their
> differences.
>
> Can you add __pfn_to_mfn() and the docs in a separate patch?

Okay.

>
>> +	pr_notice("p2m virtual area at %p, size is %lx\n", vm.addr, vm.size);
>
> pr_info().
>
>> @@ -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.

>
>>   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.


Juergen
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