linux-kernel - Re: [RFC PATCH 00/42] Sharing KVM TDP to IOMMU

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Message-ID: <ZW4AeZfCYgv6zcy4@google.com>
Date:   Mon, 4 Dec 2023 08:38:17 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Yan Zhao <yan.y.zhao@...el.com>, iommu@...ts.linux.dev,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        alex.williamson@...hat.com, pbonzini@...hat.com, joro@...tes.org,
        will@...nel.org, robin.murphy@....com, kevin.tian@...el.com,
        baolu.lu@...ux.intel.com, dwmw2@...radead.org, yi.l.liu@...el.com
Subject: Re: [RFC PATCH 00/42] Sharing KVM TDP to IOMMU

On Mon, Dec 04, 2023, Jason Gunthorpe wrote:
> On Sat, Dec 02, 2023 at 05:12:11PM +0800, Yan Zhao wrote:
> > In this series, term "exported" is used in place of "shared" to avoid
> > confusion with terminology "shared EPT" in TDX.
> > 
> > The framework contains 3 main objects:
> > 
> > "KVM TDP FD" object - The interface of KVM to export TDP page tables.
> >                       With this object, KVM allows external components to
> >                       access a TDP page table exported by KVM.
> 
> I don't know much about the internals of kvm, but why have this extra
> user visible piece?

That I don't know, I haven't looked at the gory details of this RFC.

> Isn't there only one "TDP" per kvm fd?

No.  In steady state, with TDP (EPT) enabled and assuming homogeneous capabilities
across all vCPUs, KVM will have 3+ sets of TDP page tables *active* at any given time:

  1. "Normal"
  2. SMM
  3-N. Guest (for L2, i.e. nested, VMs)

The number of possible TDP page tables used for nested VMs is well bounded, but
since devices obviously can't be nested VMs, I won't bother trying to explain the
the various possibilities (nested NPT on AMD is downright ridiculous).

Nested virtualization aside, devices are obviously not capable of running in SMM
and so they all need to use the "normal" page tables.

I highlighted "active" above because if _any_ memslot is deleted, KVM will invalidate
*all* existing page tables and rebuild new page tables as needed.  So over the
lifetime of a VM, KVM could theoretically use an infinite number of page tables.