linux-kernel - Re: [PATCH v3 19/24] KVM: x86: Introduce per-VM external cache for splitting

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Message-ID: <1cf13a8929e7087087fdafad09ec736f204eb1a7.camel@intel.com>
Date: Wed, 21 Jan 2026 19:39:36 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "seanjc@...gle.com" <seanjc@...gle.com>, "Huang, Kai"
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Subject: Re: [PATCH v3 19/24] KVM: x86: Introduce per-VM external cache for
 splitting

On Wed, 2026-01-21 at 09:30 -0800, Sean Christopherson wrote:
> I *LOVE* the core idea (seriously, this made my week), though I think we should
> take it a step further and _immediately_ do DPAMT maintenance on allocation.
> I.e. do tdx_pamt_get() via tdx_alloc_control_page() when KVM tops up the S-EPT
> SP cache instead of waiting until KVM links the SP.  Then KVM doesn't need to
> track PAMT pages except for memory that is mapped into a guest, and we end up
> with better symmetry and more consistency throughout TDX.  E.g. all pages that
> KVM allocates and gifts to the TDX-Module will allocated and freed via the same
> TDX APIs.
> 
> Absolute worst case scenario, KVM allocates 40 (KVM's SP cache capacity) PAMT
> entries per-vCPU that end up being free without ever being gifted to the TDX-Module.
> But I doubt that will be a problem in practice, because odds are good the adjacent
> pages/pfns will already have been consumed, i.e. the "speculative" allocation is
> really just bumping the refcount.  And _if_ it's a problem, e.g. results in too
> many wasted DPAMT entries, then it's one we can solve in KVM by tuning the cache
> capacity to less aggresively allocate DPAMT entries.

It doesn't sound like much impact. Especially given we earlier considered
installing DPAMT for all TDX capable memory to try to simplify things.

> 
> I'll send compile-tested v4 for the DPAMT series later today (I think I can get
> it out today), as I have other non-trival feedback that I've accumulated when
> going through the patches.

Interesting idea. I have a local branch with the rest of the feedback and a few
other tweaks. Anything I can do do help?