Re: [PATCH v1 2/2] xen: mapcache: Fix unmapping of first entries in buckets

["Alex Bennée" <alex.bennee@linaro.org>]

"Edgar E. Iglesias" <edgar.iglesias@gmail.com> writes:

> On Thu, Jul 4, 2024 at 9:48 PM Edgar E. Iglesias <edgar.iglesias@amd.com> wrote:
>
>  On Thu, Jul 04, 2024 at 05:44:52PM +0100, Alex Bennée wrote:
>  > Anthony PERARD <anthony.perard@vates.tech> writes:
>  > 
>  > > On Tue, Jul 02, 2024 at 12:44:21AM +0200, Edgar E. Iglesias wrote:
>  > >> From: "Edgar E. Iglesias" <edgar.iglesias@amd.com>
>  > >> 
>  > >> This fixes the clobbering of the entry->next pointer when
>  > >> unmapping the first entry in a bucket of a mapcache.
>  > >> 
>  > >> Fixes: 123acd816d ("xen: mapcache: Unmap first entries in buckets")
>  > >> Reported-by: Anthony PERARD <anthony.perard@vates.tech>
>  > >> Signed-off-by: Edgar E. Iglesias <edgar.iglesias@amd.com>
>  > >> ---
>  > >>  hw/xen/xen-mapcache.c | 12 +++++++++++-
>  > >>  1 file changed, 11 insertions(+), 1 deletion(-)
>  > >> 
>  > >> diff --git a/hw/xen/xen-mapcache.c b/hw/xen/xen-mapcache.c
>  > >> index 5f23b0adbe..18ba7b1d8f 100644
>  > >> --- a/hw/xen/xen-mapcache.c
>  > >> +++ b/hw/xen/xen-mapcache.c
>  > >> @@ -597,7 +597,17 @@ static void xen_invalidate_map_cache_entry_unlocked(MapCache *mc,
>  > >>          pentry->next = entry->next;
>  > >>          g_free(entry);
>  > >>      } else {
>  > >> -        memset(entry, 0, sizeof *entry);
>  > >> +        /*
>  > >> +         * Invalidate mapping but keep entry->next pointing to the rest
>  > >> +         * of the list.
>  > >> +         *
>  > >> +         * Note that lock is already zero here, otherwise we don't unmap.
>  > >> +         */
>  > >> +        entry->paddr_index = 0;
>  > >> +        entry->vaddr_base = NULL;
>  > >> +        entry->valid_mapping = NULL;
>  > >> +        entry->flags = 0;
>  > >> +        entry->size = 0;
>  > >
>  > > This kind of feels like mc->entry should be an array of pointer rather
>  > > than an array of MapCacheEntry but that seems to work well enough and
>  > > not the first time entries are been cleared like that.
>  > 
>  > The use of a hand rolled list is a bit of a concern considering QEMU and
>  > Glib both provide various abstractions used around the rest of the code
>  > base. The original patch that introduces the mapcache doesn't tell me
>  > much about access patterns for the cache, just that it is trying to
>  > solve memory exhaustion issues with lots of dynamic small mappings.
>  > 
>  > Maybe a simpler structure is desirable?
>  > 
>  > We also have an interval tree implementation ("qemu/interval-tree.h") if
>  > what we really want is a sorted tree of memory that can be iterated
>  > locklessly.
>  > 
>
>  Yes, it would be interesting to benchmark other options.
>  I agree that we should at minimum reuse existing lists/hash tables.
>
>  We've also had some discussions around removing it partially or alltogether but
>  there are some concerns around that. We're going to need something to
>  keep track of grants. For 32-bit hosts, it's a problem to exhaust virtual
>  address-space if mapping all of the guest (are folks still using 32-bit hosts?).
>  There may be other issues aswell.
>
>  Some benefits are that we'll remove some of the complexity and latency for mapping
>  and unmapping stuff continously.
>
> One more thing I forgot to add is that IMO, these larger longer term
> changes should not block this tiny bugfix...

Agreed.

>
> Cheers,
> Edgar 

-- 
Alex Bennée
Virtualisation Tech Lead @ Linaro