On 07.07.2023 18:07, Alejandro Vallejo wrote: > --- a/xen/include/xen/mm.h > +++ b/xen/include/xen/mm.h > @@ -31,6 +31,17 @@ > * (i.e. all devices assigned to) a guest share a single DMA address space > * and, by default, Xen will ensure dfn == pfn. > * > + * pdx: Page InDeX > + * Indices into the frame table holding the per-page's book-keeping > + * metadata. A compression scheme may be used, so there's a possibly non > + * identity mapping between valid(mfn) <-> valid(pdx). See the comments > + * in pdx.c for an in-depth explanation of that mapping. This also has a > + * knock-on effect on the directmap, as "compressed" pfns have no > + * corresponding mapped frames.
Didn't you mean to keep the directmap part optional, which would call for saying "may" here (twice)? Yet then ... > --- a/xen/include/xen/pdx.h > +++ b/xen/include/xen/pdx.h > @@ -1,6 +1,73 @@ > #ifndef __XEN_PDX_H__ > #define __XEN_PDX_H__ > > +/* > + * PDX (Page inDeX) > + * > + * This file deals with optimisations pertaining to frame table and > + * directmap indexing, A pdx is an index into the frame table, which > + * typically also means an index into the directmap[1]. However, having an > + * identity relationship between mfn and pdx could waste copious amounts of > + * memory in empty frame table entries and page tables. There are some > + * techniques to bring memory wastage down. > + * > + * [1] Some ports apply further modifications to a pdx before indexing the > + * directmap. This doesn't change the fact that the same compression > + * present in the frame table is also present in the directmap > + * whenever said map is present. .. you mention it here as non-optional as well. Didn't you tell me that Arm doesn't use compressed indexes into the directmap? Jan
