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Re: [Xen-devel] [RFC][Patches] Xen 1GB Page Table Support

To: George Dunlap <George.Dunlap@xxxxxxxxxxxxx>, "Huang2, Wei" <Wei.Huang2@xxxxxxx>
Subject: Re: [Xen-devel] [RFC][Patches] Xen 1GB Page Table Support
From: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Date: Wed, 18 Mar 2009 17:32:58 +0000
Cc: Tim Deegan <Tim.Deegan@xxxxxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>
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I'm not sure about putting this in for 3.4 unless there's a significant
performance win.

 -- Keir

On 18/03/2009 17:20, "George Dunlap" <George.Dunlap@xxxxxxxxxxxxx> wrote:

> Thanks for doing this work, Wei -- especially all the extra effort for
> the PoD integration.
> 
> One question: How well would you say you've tested the PoD
> functionality?  Or to put it the other way, how much do I need to
> prioritize testing this before the 3.4 release?
> 
> It wouldn't be a bad idea to do as you suggested, and break things
> into 2 meg pages for the PoD case.  In order to take the best
> advantage of this in a PoD scenario, you'd need to have a balloon
> driver that could allocate 1G of continuous *guest* p2m space, which
> seems a bit optimistic at this point...
> 
>  -George
> 
> 2009/3/18 Huang2, Wei <Wei.Huang2@xxxxxxx>:
>> Current Xen supports 2MB super pages for NPT/EPT. The attached patches
>> extend this feature to support 1GB pages. The PoD (populate-on-demand)
>> introduced by George Dunlap made P2M modification harder. I tried to
>> preserve existing PoD design by introducing a 1GB PoD cache list.
>> 
>> 
>> 
>> Note that 1GB PoD can be dropped if we don't care about 1GB when PoD is
>> enabled. In this case, we can just split 1GB PDPE into 512x2MB PDE entries
>> and grab pages from PoD super list. That can pretty much make
>> 1gb_p2m_pod.patch go away.
>> 
>> 
>> 
>> Any comment/suggestion on design idea will be appreciated.
>> 
>> 
>> 
>> Thanks,
>> 
>> 
>> 
>> -Wei
>> 
>> 
>> 
>> 
>> 
>> The following is the description:
>> 
>> === 1gb_tools.patch ===
>> 
>> Extend existing setup_guest() function. Basically, it tries to allocate 1GB
>> pages whenever available. If this request fails, it falls back to 2MB. If
>> both fail, then 4KB pages will be used.
>> 
>> 
>> 
>> === 1gb_p2m.patch ===
>> 
>> * p2m_next_level()
>> 
>> Check PSE bit of L3 page table entry. If 1GB is found (PSE=1), we split 1GB
>> into 512 2MB pages.
>> 
>> 
>> 
>> * p2m_set_entry()
>> 
>> Configure the PSE bit of L3 P2M table if page order == 18 (1GB).
>> 
>> 
>> 
>> * p2m_gfn_to_mfn()
>> 
>> Add support for 1GB case when doing gfn to mfn translation. When L3 entry is
>> marked as POPULATE_ON_DEMAND, we call 2m_pod_demand_populate(). Otherwise,
>> we do the regular address translation (gfn ==> mfn).
>> 
>> 
>> 
>> * p2m_gfn_to_mfn_current()
>> 
>> This is similar to p2m_gfn_to_mfn(). When L3 entry s marked as
>> POPULATE_ON_DEMAND, it demands a populate using p2m_pod_demand_populate().
>> Otherwise, it does a normal translation. 1GB page is taken into
>> consideration.
>> 
>> 
>> 
>> * set_p2m_entry()
>> 
>> Request 1GB page
>> 
>> 
>> 
>> * audit_p2m()
>> 
>> Support 1GB while auditing p2m table.
>> 
>> 
>> 
>> * p2m_change_type_global()
>> 
>> Deal with 1GB page when changing global page type.
>> 
>> 
>> 
>> === 1gb_p2m_pod.patch ===
>> 
>> * xen/include/asm-x86/p2m.h
>> 
>> Minor change to deal with PoD. It separates super page cache list into 2MB
>> and 1GB lists. Similarly, we record last gpfn of sweeping for both 2MB and
>> 1GB.
>> 
>> 
>> 
>> * p2m_pod_cache_add()
>> 
>> Check page order and add 1GB super page into PoD 1GB cache list.
>> 
>> 
>> 
>> * p2m_pod_cache_get()
>> 
>> Grab a page from cache list. It tries to break 1GB page into 512 2MB pages
>> if 2MB PoD list is empty. Similarly, 4KB can be requested from super pages.
>> The breaking order is 2MB then 1GB.
>> 
>> 
>> 
>> * p2m_pod_cache_target()
>> 
>> This function is used to set PoD cache size. To increase PoD target, we try
>> to allocate 1GB from xen domheap. If this fails, we try 2MB. If both fail,
>> we try 4KB which is guaranteed to work.
>> 
>> 
>> 
>> To decrease the target, we use a similar approach. We first try to free 1GB
>> pages from 1GB PoD cache list. If such request fails, we try 2MB PoD cache
>> list. If both fail, we try 4KB list.
>> 
>> 
>> 
>> * p2m_pod_zero_check_superpage_1gb()
>> 
>> This adds a new function to check for 1GB page. This function is similar to
>> p2m_pod_zero_check_superpage_2mb().
>> 
>> 
>> 
>> * p2m_pod_zero_check_superpage_1gb()
>> 
>> We add a new function to sweep 1GB page from guest memory. This is the same
>> as p2m_pod_zero_check_superpage_2mb().
>> 
>> 
>> 
>> * p2m_pod_demand_populate()
>> 
>> The trick of this function is to do remap_and_retry if p2m_pod_cache_get()
>> fails. When p2m_pod_get() fails, this function will splits p2m table entry
>> into smaller ones (e.g. 1GB ==> 2MB or 2MB ==> 4KB). That can guarantee
>> populate demands always work.
>> 
>> 
>> 
>> 
>> 
>> _______________________________________________
>> Xen-devel mailing list
>> Xen-devel@xxxxxxxxxxxxxxxxxxx
>> http://lists.xensource.com/xen-devel
>> 
>> 



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