Re: [PATCH 4/6] x86/virt/tdx: Add extra memory to TDX module for the extensions

[Kiryl Shutsemau <kas@kernel.org>]

On Thu, Sep 03, 2026 at 06:40:04PM +0800, Xu Yilun wrote:
> On Wed, Sep 02, 2026 at 11:54:25AM +0100, Kiryl Shutsemau wrote:
> > On Fri, Aug 28, 2026 at 03:46:16PM +0800, Xu Yilun wrote:
> > > > > > +	page = alloc_contig_pages(required_pages, GFP_KERNEL, numa_mem_id(),
> > > > > > +				  &node_online_map);
> > > > > 
> > > > > Why contiguous? TDH.EXT.MEM.ADD takes a list of page addresses and the loop
> > > > > below writes every one of them out separately.
> > > > > 
> > > > > alloc_pages_bulk() fits the chunking that is already here, and a short
> > > > > return can be handled per chunk. alloc_contig_pages() isolates and migrates
> > > > > to get its range and fails TDX init outright when it cannot find one. PAMT
> > > > 
> > > > Yeah, this is not the ABI requirement, but the kernel's consideration. A
> > > > brief reasoning in the commit log: avoiding permanent memory fragmentation
> > > > and buddy allocator efficiency loss.
> > > > 
> > > > Also there is some discussion:
> > > > 
> > > > https://lore.kernel.org/all/167d9540-2d9a-4367-bc68-b96494bc4044@intel.com/
> > > > 
> > > > TL;DR
> > > >   - The memory will never return to the kernel.
> > > >   - There is chance that this tens of megabytes will fragment tens of
> > > >     gigabytes of memory forever.
> > > >   - The chance of fragmentation is actually low since at boot up, but
> > > >     let the buddy allocator take care of these never-returned memory
> > > >     is not necessary and lowers its efficiency.
> > > 
> > > Hi Kiryl & David:
> > > 
> > > I see there is another suggestion that the whole memory adding process
> > > could be a little simpler if we allocate & add pages 4k by 4k [1],
> > > rather than one-time pre-allocation. The concern of this alternative is,
> > > as said above, memory fragmentation.
> > > 
> > > [1] https://lore.kernel.org/lkml/f48b83feb2ee1d3c88b5a1627cf35b4b282d3f90.camel@intel.com/
> > > 
> > > And I've realized the memory fragmentation discussion is not actually
> > > closed in previous thread [2]. We need more input.
> > > 
> > > [2] https://lore.kernel.org/all/167d9540-2d9a-4367-bc68-b96494bc4044@intel.com/
> > > 
> > > Let me give a brief overview of the problem:
> > > 
> > > Intel TDX (Trust Domain Extensions) is a feature for confidential
> > > computing. A secure firmware called "TDX module" runs in an isolated
> > > environment to provide services about security.
> > > 
> > > In Linux, the host initializes TDX module at boot up time
> > > (subsys_initcall()). During the initialization, the host must donate
> > > tens of mega bytes physical memory (35M ~ 110M in the forseeable
> > > future) to the TDX module. These memory will *never be revoked* cause
> > > the TDX Module initialization is a one way path.
> > > 
> > > The TDX Module doesn't require this memory be physically contiguous. But
> > > the kernel side concern is if we do PAGE_SIZE allocation, it may
> > > permanently fragment memory regions, stop them from allocating 2M huge
> > > pages. In worst case, ~50G (110M * 512) memory regions affacted.
> > > 
> > > So is the physically contiguous allocation really a better choice here?
> > > We appreciate inputs from mm folks. Thanks!
> > 
> > What matters for fragmentation is not contiguity, it is how many
> > pageblocks are left partially occupied by unmovable pages that are never
> > freed.
> > 
> > So you can ask one pageblock at a time with
> > 
> > 	page = alloc_pages(GFP_KERNEL | __GFP_NOWARN, order);
> > 
> > with fallback to lower order if you must.
> > 
> > It also fits the ABI: pageblock_order is 9 on x86, i.e. 512 pages, which
> > is exactly TDX_HPA_LIST_MAX_NR_PAGES. One allocation is one full HPA list
> > is one TDH.EXT.MEM.ADD, so the allocation loop and the chunking loop
> > become the same loop.
> > 
> > But alloc_contig_pages() might be a good enough approximation for
> > per-pageblock allocation if we do it during the boot when fragmentation
> > is low.
> 
> IIUC, you mean alloc_contig_pages() also gives good de-fragmentation
> that we need. But it would be slightly easier to fail cause it requires
> extra contiguity that we don't need.

alloc_contig_pages() can be more expensive than needed (or fail) since
you ask for the full allocation size to be contiguous, where you should
be okay with a set of pageblocks regardless where they are relative to
each other.

> Multiple alloc_pages(order-9) meets our requirement exactly but the
> falling back to lower order may create more fragments. And we can do
> this because of the ABI definition - an HPA_LIST could happen to hold
> an entire pageblock.
> 
> If I have to choose, I prefer alloc_contig_pages(). It doesn't have to
> depend on HPA_LIST ABI details.

As I said before, as long as you do it once during the boot, it should
be good enough.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov