Re: [Qemu-devel] [PATCH] xen-mapcache: don't unmap locked entry during mapcache invalidation

[Andres Lagar-Cavilla]

> On Thu, 15 Mar 2012, Julien Grall wrote:
>> When an IOREQ_TYPE_INVALIDATE is sent to QEMU, it invalidates all entry
>> of the map cache even if it's locked.
>>
>> QEMU is not able to know that entry was invalidated, so when an IO
>> access is requested a segfault occured.
>
> The problem here is the long term mappings in QEMU that cannot easily be
> re-created.
> I am not sure whether this can cause any trouble to things like
> xenpaging.

This is a performance optimization for xenpaging: it cannot page out pages
that are mapped by qemu, so it asks qemu to drop the mappings. If denied
(partially or fully) xenpaging won't crash, and the VM won't crash.
xenpaging will just have to keep trying to find other candidates for
paging out.

Andres

>
>
>> Signed-off-by: Julien Grall <address@hidden>
>> ---
>>   xen-mapcache.c |    3 +++
>>   1 files changed, 3 insertions(+), 0 deletions(-)
>>
>> diff --git a/xen-mapcache.c b/xen-mapcache.c
>> index 585b559..6e7e5ab 100644
>> --- a/xen-mapcache.c
>> +++ b/xen-mapcache.c
>> @@ -370,6 +370,9 @@ void xen_invalidate_map_cache(void)
>>               continue;
>>           }
>> +    if (entry->lock > 0)
>> +        continue;
>> +
>>           if (munmap(entry->vaddr_base, entry->size) != 0) {
>>               perror("unmap fails");
>>               exit(-1);
>
> CODING_STYLE
>