linux-kernel - Re: [PATCH v7 10/20] x86/virt/tdx: Use all system memory when initializing TDX module as TDX memory

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Date:   Thu, 24 Nov 2022 10:26:20 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Kai Huang <kai.huang@...el.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, linux-mm@...ck.org, seanjc@...gle.com,
        pbonzini@...hat.com, dave.hansen@...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, 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
Subject: Re: [PATCH v7 10/20] x86/virt/tdx: Use all system memory when
 initializing TDX module as TDX memory

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).