Date:   Wed, 23 Nov 2022 17:50:37 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Kai Huang <kai.huang@...el.com>, <linux-kernel@...r.kernel.org>,
        <kvm@...r.kernel.org>
CC:     <linux-mm@...ck.org>, <seanjc@...gle.com>, <pbonzini@...hat.com>,
        <dave.hansen@...el.com>, <dan.j.williams@...el.com>,
        <rafael.j.wysocki@...el.com>, <kirill.shutemov@...ux.intel.com>,
        <ying.huang@...el.com>, <reinette.chatre@...el.com>,
        <len.brown@...el.com>, <tony.luck@...el.com>,
        <peterz@...radead.org>, <ak@...ux.intel.com>,
        <isaku.yamahata@...el.com>, <chao.gao@...el.com>,
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        <bagasdotme@...il.com>, <sagis@...gle.com>, <imammedo@...hat.com>,
        <kai.huang@...el.com>
Subject: RE: [PATCH v7 10/20] x86/virt/tdx: Use all system memory when
 initializing TDX module as TDX memory

Kai Huang wrote:
> TDX reports a list of "Convertible Memory Region" (CMR) to indicate all
> memory regions that can possibly be used by the TDX module, but they are
> not automatically usable to the TDX module.  As a step of initializing
> the TDX module, the kernel needs to choose a list of memory regions (out
> from convertible memory regions) that the TDX module can use and pass
> those regions to the TDX module.  Once this is done, those "TDX-usable"
> memory regions are fixed during module's lifetime.  No more TDX-usable
> memory can be added to the TDX module after that.
> 
> The initial support of TDX guests will only allocate TDX guest memory
> from the global page allocator.  To keep things simple, this initial
> implementation simply guarantees all pages in the page allocator are TDX
> memory.  To achieve this, use all system memory in the core-mm at the
> time of initializing the TDX module as TDX memory, and at the meantime,
> refuse to add any non-TDX-memory in the memory hotplug.
> 
> Specifically, walk through all memory regions managed by memblock and
> add them to a global list of "TDX-usable" memory regions, which is a
> fixed list after the module initialization (or empty if initialization
> fails).  To reject non-TDX-memory in memory hotplug, add an additional
> check in arch_add_memory() to check whether the new region is covered by
> any region in the "TDX-usable" memory region list.
> 
> Note this requires all memory regions in memblock are TDX convertible
> memory when initializing the TDX module.  This is true in practice if no
> new memory has been hot-added before initializing the TDX module, since
> in practice all boot-time present DIMM is TDX convertible memory.  If
> any new memory has been hot-added, then initializing the TDX module will
> fail due to that memory region is not covered by CMR.
> 
> This can be enhanced in the future, i.e. by allowing adding non-TDX
> memory to a separate NUMA node.  In this case, the "TDX-capable" nodes
> and the "non-TDX-capable" nodes can co-exist, but the kernel/userspace
> needs to guarantee memory pages for TDX guests are always allocated from
> the "TDX-capable" nodes.
> 
> Note TDX assumes convertible memory is always physically present during
> machine's runtime.  A non-buggy BIOS should never support hot-removal of
> any convertible memory.  This implementation doesn't handle ACPI memory
> removal but depends on the BIOS to behave correctly.
> 
> Signed-off-by: Kai Huang <kai.huang@...el.com>
> ---
> 
> v6 -> v7:
>  - Changed to use all system memory in memblock at the time of
>    initializing the TDX module as TDX memory
>  - Added memory hotplug support
> 
> ---
>  arch/x86/Kconfig            |   1 +
>  arch/x86/include/asm/tdx.h  |   3 +
>  arch/x86/mm/init_64.c       |  10 ++
>  arch/x86/virt/vmx/tdx/tdx.c | 183 ++++++++++++++++++++++++++++++++++++
>  4 files changed, 197 insertions(+)
> 
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index dd333b46fafb..b36129183035 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -1959,6 +1959,7 @@ config INTEL_TDX_HOST
>  	depends on X86_64
>  	depends on KVM_INTEL
>  	depends on X86_X2APIC
> +	select ARCH_KEEP_MEMBLOCK
>  	help
>  	  Intel Trust Domain Extensions (TDX) protects guest VMs from malicious
>  	  host and certain physical attacks.  This option enables necessary TDX
> diff --git a/arch/x86/include/asm/tdx.h b/arch/x86/include/asm/tdx.h
> index d688228f3151..71169ecefabf 100644
> --- a/arch/x86/include/asm/tdx.h
> +++ b/arch/x86/include/asm/tdx.h
> @@ -111,9 +111,12 @@ static inline long tdx_kvm_hypercall(unsigned int nr, unsigned long p1,
>  #ifdef CONFIG_INTEL_TDX_HOST
>  bool platform_tdx_enabled(void);
>  int tdx_enable(void);
> +bool tdx_cc_memory_compatible(unsigned long start_pfn, unsigned long end_pfn);
>  #else	/* !CONFIG_INTEL_TDX_HOST */
>  static inline bool platform_tdx_enabled(void) { return false; }
>  static inline int tdx_enable(void)  { return -ENODEV; }
> +static inline bool tdx_cc_memory_compatible(unsigned long start_pfn,
> +		unsigned long end_pfn) { return true; }
>  #endif	/* CONFIG_INTEL_TDX_HOST */
>  
>  #endif /* !__ASSEMBLY__ */
> diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
> index 3f040c6e5d13..900341333d7e 100644
> --- a/arch/x86/mm/init_64.c
> +++ b/arch/x86/mm/init_64.c
> @@ -55,6 +55,7 @@
>  #include <asm/uv/uv.h>
>  #include <asm/setup.h>
>  #include <asm/ftrace.h>
> +#include <asm/tdx.h>
>  
>  #include "mm_internal.h"
>  
> @@ -968,6 +969,15 @@ int arch_add_memory(int nid, u64 start, u64 size,
>  	unsigned long start_pfn = start >> PAGE_SHIFT;
>  	unsigned long nr_pages = size >> PAGE_SHIFT;
>  
> +	/*
> +	 * For now if TDX is enabled, all pages in the page allocator
> +	 * must be TDX memory, which is a fixed set of memory regions
> +	 * that are passed to the TDX module.  Reject the new region
> +	 * if it is not TDX memory to guarantee above is true.
> +	 */
> +	if (!tdx_cc_memory_compatible(start_pfn, start_pfn + nr_pages))
> +		return -EINVAL;

arch_add_memory() does not add memory to the page allocator.  For
example, memremap_pages() uses arch_add_memory() and explicitly does not
release the memory to the page allocator. This check belongs in
add_memory_resource() to prevent new memory that violates TDX from being
onlined. Hopefully there is also an option to disable TDX from the
kernel boot command line to recover memory-hotplug without needing to
boot into the BIOS to toggle TDX.

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