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Some questions about x86/xen pmap

Hi lists,

I have some questions regarding the pmap of x86/xen, mainly for practical reasons; I am not very familiar with it.

Just to set the background, to make Xen save/restore/migrate work under a NetBSD domU, I have to do some slight modifications to the pmap, for two reasons: - Xen (and xentools) do not handle alternative recursive mappings (the APDP entries in our PD). - domains should not keep references to machine frame number (real physical page numbers) between save/restore, as they could change (especially when migrating VMs) without the domain noticing it.

I currently handle these issues that way:

- just before save, I am iterating through the pmap's to check that the PDIR_SLOT_APTE is set to 0. Otherwise, set it to 0.

- functions using MFNs are protected with rwlock(9) routines. Prior to using a MFN, the thread has to acquire a reader lock, and release it when it has finished with it. The suspension thread acquires a writer lock, so that every other thread that wants to manipulate MFNs have to wait for the suspension to release the writer, which is done during resume when it is deemed safe.

Questions are following:

- apart from the pmap_map_ptes() routines, which are used to map an arbitrary pmap inside the current one, is there another use of the APDP slot? I can't find any, but I may have missed it during my grep/read stuff.

- in pmap_unmap_ptes() (used to unlock the pmap used earlier by pmap_map_ptes()), the APDP is only unmapped when MULTIPROCESSOR is defined. While it is now the case for GENERIC, we do not have SMP support inside port-xen currently. Why is it necessary to empty the entry for MP and not for UP?

- what would be the fastest way to iterate through pmaps? My current code is inspired from the one found in pmap_extract(), but it requires to map/unmap each pmap to check for non-0 APDP slots. As they are not that common, this may be costly when the pmap list is lengthy. I tried to access the PD of each pmap through their pm_pdir member, but it usually ends badly with a fault exception in some cases.

- rwlock(9) clearly states that callers must not /recursively/ acquire read locks. Is a thread allowed to acquire a reader lock multiple times, provided it is not done recursively?

- the read/write locking is inspired from the one found in Solaris. If you have suggestion on doing the locking another way, feel free to suggest. I used rwlock(9) as it allows multiple readers, hence having minimal overhead in our case, while having write-preferring when domain is suspending.

Many thanks for your patience while reading, I tend to be a bit wordy in my mails.


Jean-Yves Migeon

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