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Date:   Thu, 24 Nov 2022 10:26:20 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Dan Williams <dan.j.williams@intel.com>
Cc:     Kai Huang <kai.huang@intel.com>, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org, linux-mm@kvack.org, seanjc@google.com,
        pbonzini@redhat.com, dave.hansen@intel.com,
        rafael.j.wysocki@intel.com, kirill.shutemov@linux.intel.com,
        ying.huang@intel.com, reinette.chatre@intel.com,
        len.brown@intel.com, tony.luck@intel.com, ak@linux.intel.com,
        isaku.yamahata@intel.com, chao.gao@intel.com,
        sathyanarayanan.kuppuswamy@linux.intel.com, bagasdotme@gmail.com,
        sagis@google.com, imammedo@redhat.com
Subject: Re: [PATCH v7 10/20] x86/virt/tdx: Use all system memory when
 initializing TDX module as TDX memory
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On Wed, Nov 23, 2022 at 05:50:37PM -0800, Dan Williams wrote:

> 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.

So I've been pushing for all this to either require: tdx=force on the
cmdline to boot-time enable, or delay all the memory allocation to the
first KVM/TDX instance being created.

That is, by default, none of this crud should ever trigger and consume
memory if you're not using TDX (most of us really).

(every machine I have loads kvm.ko unconditionally -- even if I never
user KVM, so kvm.ko load time is not a valid point in time to do TDX
enablement).