[Xen-devel] [PATCH][UPDATE]Remove lock on guest table walk

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

>From: Tim Deegan
>Sent: 2008年2月21日 22:13
>Hi,
>
>So, the idea seems sound, and avoids the shadow lock altogether on a
>bunch more pagefaults, which is nice. 
>
>I think that since PV pagetables are guaranteed to be read-only above
>l1e, the guest_map_l1e and guest_get_eff_l1e functions can be 
>allowed to
>drop the shadow lock and call guest_walk_tables with shadow_op == 0.
>That would mean that there are no callers left setting shadow_op to 1,
>and then the shadow_op argument (and the whole mechanism for calling
>remove_write_access from guest_walk_tables) could be removed.

Thanks for clarification.

>
>I think that in guest_walk_tables, you need to add an rmb() after
>reading the version number to stop the compiler from hoisting the
>pagetable reads to before the version read.

Yes, my overlook and thanks for pointing out.

>
>Coding nits:
> - I'd like to see the "version" field called something more
>   descriptive, and moved into the shadow-specific domain state, 
>   since HAP won't be using it.

Done and renamed to gtable_dirty_version. Sorry if this is still not a
good name and please feel free help polish a better name. :-)

> - In shadow_check_gwalk, maybe return 1 at the top of the function if
>   the version number hasn't changed, rather than putting most of the 
>   function inside an if()?

Agree.

> - You've added a second "not a shadow_fault" printout without removing
>   the first.

The first one is branched with lock held, and some audit functions also
required lock held. To differentiate, I put a new label. Now I just removed
the new label since only few places use it.

> - We can remove the comment at the top of multi.c about reworking the
>   guest_walk TLB flush logic. :)

Done.

Then updated version attached, and please help check again.

Thanks,
Kevin

Attachment: gwalk_no_lock.patch
Description: gwalk_no_lock.patch

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