Hi Jan,
On 4/2/2024 3:05 PM, Jan Beulich wrote:
On 29.03.2024 06:11, Henry Wang wrote:
On 3/12/2024 1:07 AM, Jan Beulich wrote:
+/*
+ * Flag to force populate physmap to use pages from domheap instead of 1:1
+ * or static allocation.
+ */
+#define XENMEMF_force_heap_alloc (1<<19)
#endif
If this is for populate_physmap only, then other sub-ops need to reject
its use.
I have to admit I'm a little wary of allocating another flag here and ...
--- a/xen/include/xen/mm.h
+++ b/xen/include/xen/mm.h
@@ -205,6 +205,8 @@ struct npfec {
#define MEMF_no_icache_flush (1U<<_MEMF_no_icache_flush)
#define _MEMF_no_scrub 8
#define MEMF_no_scrub (1U<<_MEMF_no_scrub)
+#define _MEMF_force_heap_alloc 9
+#define MEMF_force_heap_alloc (1U<<_MEMF_force_heap_alloc)
#define _MEMF_node 16
#define MEMF_node_mask ((1U << (8 * sizeof(nodeid_t))) - 1)
#define MEMF_node(n) ((((n) + 1) & MEMF_node_mask) << _MEMF_node)
... here - we don't have that many left. Since other sub-ops aren't
intended to support this flag, did you consider adding another (perhaps
even arch-specific) sub-op instead?
While revisiting this comment when trying to come up with a V3, I
realized adding a sub-op here in the same level as
XENMEM_populate_physmap will basically duplicate the function
populate_physmap() with just the "else" (the non-1:1 allocation) part,
also a similar xc_domain_populate_physmap_exact() & co will be needed
from the toolstack side to call the new sub-op. So I am having the
concern of the duplication of code and not sure if I understand you
correctly. Would you please elaborate a bit more or clarify if I
understand you correctly? Thanks!
Well, the goal is to avoid both code duplication and introduction of a new,
single-use flag. The new sub-op suggestion, I realize now, would mainly have
helped with avoiding the new flag in the public interface. That's still
desirable imo. Internally, have you checked which MEMF_* are actually used
by populate_physmap()? Briefly looking, e.g. MEMF_no_dma and MEMF_no_refcount
aren't. It therefore would be possible to consider re-purposing one that
isn't (likely to be) used there. Of course doing so requires care to avoid
passing that flag down to other code (page_alloc.c functions in particular),
where the meaning would be the original one.
I think you made a good point, however, to be honest I am not sure about
the repurposing flags such as MEMF_no_dma and MEMF_no_refcount, because
I think the name and the purpose of the flag should be clear and
less-misleading. Reusing either one for another meaning, namely forcing
a non-heap allocation in populate_physmap() would be confusing in the
future. Also if one day these flags will be needed in
populate_physmap(), current repurposing approach will lead to a even
confusing code base.
I also do agree very much that we need to also avoid the code
duplication, so compared to other two suggested approach, adding a new
MEMF would be the cleanest solution IMHO, as it is just one bit and MEMF
flags are not added very often.
I would also curious what the other common code maintainers will say on
this issue: @Andrew, @Stefano, @Julien, any ideas? Thanks!
Kind regards,
Henry
Jan
