Re: [PATCH v6 08/11] x86/tdx: Add APIs to support Dynamic PAMT ops from KVM's fault path

[Kiryl Shutsemau]

On Mon, May 25, 2026 at 07:35:12PM -0700, Rick Edgecombe wrote:
> When handling an EPT violation, KVM holds a spinlock while manipulating
> the EPT. Before entering the spinlock it doesn't know how many EPT page
> tables will need to be installed or whether a huge page will be used. For
> this reason it allocates a worst case number of page tables that it might
> need as part of servicing the EPT violation.
> 
> Under Dynamic PAMT these pre-allocated pages will potentially need to have
> Dynamic PAMT backing pages installed for them. KVM already has helpers to
> manage topping up page caches before taking the MMU lock, but they cannot be
> passed from KVM to arch/x86 code.
> 
> The problem of how and when to install the DPAMT backing pages for the
> pages given to the TDX module during the fault path has had a lot of
> design attempts.
>  - Extracting KVM's MMU caches requires too much inlined code added to
>    headers.
>  - A few varieties of installing Dynamic PAMT backing when allocating the
>    S-EPT page tables. [0][1]
>  - Using mempool_t to transfer the pages between KVM and arch/x86 doesn't
>    work because it is the component is designed more around maintaining a
>    pool of pages, rather than topping up a continually drained cache.
> 
> So don't do these as they all had various problems. Instead just create a
> small simple data structure to use for handing a pre-allocated list of
> pages between KVM and arch/x86 code. Model this on KVM's existing MMU
> memory caches.
> 
> Add a tdx_pamt_cache arg to tdx_pamt_get() so it can draw pages from a
> cache when needed. Not all DPAMT page installations will happen under
> spinlock, for example control pages. So have tdx_pamt_get() maintain the
> existing behavior of allocating from the page allocator when NULL is
> passed for the struct tdx_pamt_cache arg. This prevents excess allocations
> for cases where it can be avoided.
> 
> Export the new helpers for KVM.
> 
> Assisted-by: GitHub Copilot:claude-opus-4-6 Claude:claude-opus-4-7
> Co-developed-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
> Link: https://lore.kernel.org/kvm/de05853257e9cc66998101943f78a4b7e6e3d741.camel@xxxxxxxxx/ [0]
> Link: https://lore.kernel.org/kvm/aYprxnSHKHUtk7pt@xxxxxxxxxx/ [1]

Reviewed-by: Kiryl Shutsemau (Meta) <kas@xxxxxxxxxx>

-- 
  Kiryl Shutsemau / Kirill A. Shutemov