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SubjectRe: [PATCH v12 12/22] x86/virt/tdx: Allocate and set up PAMTs for TDMRs
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@intel.com>
> Reviewed-by: Isaku Yamahata <isaku.yamahata@intel.com>
> Reviewed-by: Dave Hansen <dave.hansen@linux.intel.com>

Reviewed-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>

--
Kiryl Shutsemau / Kirill A. Shutemov

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