linux-kernel - Re: [PATCH v12 12/22] x86/virt/tdx: Allocate and set up PAMTs for TDMRs

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Message-ID: <20230627095152.zmeb2djphpboo5ya@box.shutemov.name>
Date:   Tue, 27 Jun 2023 12:51:52 +0300
From:   kirill.shutemov@...ux.intel.com
To:     Kai Huang <kai.huang@...el.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-mm@...ck.org, x86@...nel.org, dave.hansen@...el.com,
        tony.luck@...el.com, peterz@...radead.org, tglx@...utronix.de,
        bp@...en8.de, mingo@...hat.com, hpa@...or.com, seanjc@...gle.com,
        pbonzini@...hat.com, david@...hat.com, dan.j.williams@...el.com,
        rafael.j.wysocki@...el.com, ashok.raj@...el.com,
        reinette.chatre@...el.com, len.brown@...el.com, ak@...ux.intel.com,
        isaku.yamahata@...el.com, ying.huang@...el.com, chao.gao@...el.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com, nik.borisov@...e.com,
        bagasdotme@...il.com, sagis@...gle.com, imammedo@...hat.com
Subject: Re: [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@...el.com>
> Reviewed-by: Isaku Yamahata <isaku.yamahata@...el.com>
> Reviewed-by: Dave Hansen <dave.hansen@...ux.intel.com>

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

-- 
  Kiryl Shutsemau / Kirill A. Shutemov