Re: [Qemu-devel] [PATCH v2 10/17] translate-all: use per-page locking in !user-mode

["Emilio G. Cota" <cota@braap.org>]

Hi Richard,

On Mon, Apr 23, 2018 at 20:18:31 -0400, Emilio G. Cota wrote:
> On Fri, Apr 13, 2018 at 17:29:20 -1000, Richard Henderson wrote:
> > On 04/05/2018 04:13 PM, Emilio G. Cota wrote:
> (snip)
> > > +struct page_collection {
> > > +    GTree *tree;
> > > +    struct page_entry *max;
> > > +};
> > 
> > I don't understand the motivation for this data structure.  Substituting one
> > tree for another does not, on the face of it, seem to be a win.
> > 
> > Given that your locking order is based on the physical address, I can
> > understand that the sequential virtual addresses that these routines are given
> > is not appropriate.  But surely you should know how many pages are involved,
> > and therefore be able to allocate a flat array to hold the PageDesc.
> > 
> > > +/*
> > > + * Lock a range of pages ([@start,@end[) as well as the pages of all
> > > + * intersecting TBs.
> > > + * Locking order: acquire locks in ascending order of page index.
> > > + */
> > 
> > I don't think I understand this either.  From whence do you wind up with a
> > range of physical addresses?
> 
> For instance in tb_invalidate_phys_page_range. We need to invalidate
> all TBs associated with a range of phys addresses.
> 
> I am not sure how an array would make things easier, since we need
> to lock the pages in the given range, as well as the pages that
> overlap with the TBs in said range (since we'll invalidate the TBs).
> For example, if we have to invalidate all TBs in the range A-E, it
> is possible that a TB in page C will overlap with page K (not in
> the original range), so we'll have to lock page K as well. All of
> this needs to be done in order, that is, A-E,K.
> 
> If we had an array, we'd have to resize the array anytime we had
> an out-of-range page, and then do a binary search in the array
> to check whether we already locked that page. At this point
> we'd be reinventing a binary tree, so it seems simpler to just
> use a tree.

Do you have any comments wrt the above (and my other replies to
your comments in this series)? I'd like to do a respin this week.

Thanks,

		Emilio