On Tue, Jun 27, 2023 at 02:12:42AM +1200, Kai Huang wrote: > The TDX module uses additional metadata to record things like which > guest "owns" a given page of memory. This metadata, referred as > Physical Address Metadata Table (PAMT), essentially serves as the > 'struct page' for the TDX module. PAMTs are not reserved by hardware > up front. They must be allocated by the kernel and then given to the > TDX module during module initialization. > > TDX supports 3 page sizes: 4K, 2M, and 1G. Each "TD Memory Region" > (TDMR) has 3 PAMTs to track the 3 supported page sizes. Each PAMT must > be a physically contiguous area from a Convertible Memory Region (CMR). > However, the PAMTs which track pages in one TDMR do not need to reside > within that TDMR but can be anywhere in CMRs. If one PAMT overlaps with > any TDMR, the overlapping part must be reported as a reserved area in > that particular TDMR. > > Use alloc_contig_pages() since PAMT must be a physically contiguous area > and it may be potentially large (~1/256th of the size of the given TDMR). > The downside is alloc_contig_pages() may fail at runtime. One (bad) > mitigation is to launch a TDX guest early during system boot to get > those PAMTs allocated at early time, but the only way to fix is to add a > boot option to allocate or reserve PAMTs during kernel boot. > > It is imperfect but will be improved on later. > > TDX only supports a limited number of reserved areas per TDMR to cover > both PAMTs and memory holes within the given TDMR. If many PAMTs are > allocated within a single TDMR, the reserved areas may not be sufficient > to cover all of them. > > Adopt the following policies when allocating PAMTs for a given TDMR: > > - Allocate three PAMTs of the TDMR in one contiguous chunk to minimize > the total number of reserved areas consumed for PAMTs. > - Try to first allocate PAMT from the local node of the TDMR for better > NUMA locality. > > Also dump out how many pages are allocated for PAMTs when the TDX module > is initialized successfully. This helps answer the eternal "where did > all my memory go?" questions. > > Signed-off-by: Kai Huang <kai.huang@xxxxxxxxx> > Reviewed-by: Isaku Yamahata <isaku.yamahata@xxxxxxxxx> > Reviewed-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx> Reviewed-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx> -- Kiryl Shutsemau / Kirill A. Shutemov
