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

[George Dunlap <George.Dunlap@xxxxxxxxxxxxx>]

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.
>
>
>
>
>
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> http://lists.xensource.com/xen-devel
>
>

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