linux-kernel - Re: [PATCH v10 107/108] KVM: x86: design documentation on TDX support of x86 KVM TDP MMU

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Message-ID: <Y19NzlQcwhV/2wl3@debian.me>
Date:   Mon, 31 Oct 2022 11:23:42 +0700
From:   Bagas Sanjaya <bagasdotme@...il.com>
To:     isaku.yamahata@...el.com
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        isaku.yamahata@...il.com, Paolo Bonzini <pbonzini@...hat.com>,
        erdemaktas@...gle.com, Sean Christopherson <seanjc@...gle.com>,
        Sagi Shahar <sagis@...gle.com>,
        David Matlack <dmatlack@...gle.com>
Subject: Re: [PATCH v10 107/108] KVM: x86: design documentation on TDX
 support of x86 KVM TDP MMU

On Sat, Oct 29, 2022 at 11:23:48PM -0700, isaku.yamahata@...el.com wrote:
> +During TDX non-root operation (i.e. guest TD), memory accesses can be qualified
> +as either shared or private, based on the value of a new SHARED bit in the Guest
> +Physical Address (GPA).  The CPU translates shared GPAs using the usual VMX EPT
> +(Extended Page Table) or "Shared EPT" (in this document), which resides in the
> +host VMM memory.  The Shared EPT is directly managed by the host VMM - the same
> +as with the current VMX.  Since guest TDs usually require I/O, and the data
> +exchange needs to be done via shared memory, thus KVM needs to use the current
> +EPT functionality even for TDs.

Strip the last "thus", so it becomes "... via shared memory, KVM needs to use ..."

> +The following depicts the relationship.
> +::
> +
> +                    KVM                             |       TDX module
> +                     |                              |           |
> +        -------------+----------                    |           |
> +        |                      |                    |           |
> +        V                      V                    |           |
> +     shared GPA           private GPA               |           |
> +  CPU shared EPT pointer  KVM private EPT pointer   |  CPU secure EPT pointer
> +        |                      |                    |           |
> +        |                      |                    |           |
> +        V                      V                    |           V
> +  shared EPT                private EPT<-------mirror----->Secure EPT
> +        |                      |                    |           |
> +        |                      \--------------------+------\    |
> +        |                                           |      |    |
> +        V                                           |      V    V
> +  shared guest page                                 |    private guest page
> +                                                    |
> +                                                    |
> +                              non-encrypted memory  |    encrypted memory
> +                                                    |
> +
> +shared EPT: CPU and KVM walk with shared GPA
> +            Maintained by the existing code
> +private EPT: KVM walks with private GPA
> +             Maintained by the twisted existing code
> +secure EPT: CPU walks with private GPA.
> +            Maintained by TDX module with TDX SEAMCALLs via hooks
> +

What about this legend below?

---- >8 ----

diff --git a/Documentation/virt/kvm/tdx-tdp-mmu.rst b/Documentation/virt/kvm/tdx-tdp-mmu.rst
index 2d91c94e6d8fd7..9ddbf44725f212 100644
--- a/Documentation/virt/kvm/tdx-tdp-mmu.rst
+++ b/Documentation/virt/kvm/tdx-tdp-mmu.rst
@@ -236,12 +236,14 @@ The following depicts the relationship.
                               non-encrypted memory  |    encrypted memory
                                                     |
 
-shared EPT: CPU and KVM walk with shared GPA
-            Maintained by the existing code
-private EPT: KVM walks with private GPA
-             Maintained by the twisted existing code
-secure EPT: CPU walks with private GPA.
-            Maintained by TDX module with TDX SEAMCALLs via hooks
+Where:
+
+  * shared EPT: CPU and KVM walk with shared GPA (maintained by the existing
+    code)
+  * private EPT: KVM walks with private GPA (maintained by the twisted existing
+    code)
+  * secure EPT: CPU walks with private GPA (maintained by TDX module with TDX
+    SEAMCALLs via hooks).
 
 
 Tracking private EPT page

> +Concurrent zapping
> +------------------
> +1. read lock
> +2. freeze the EPT entry (atomically set the value to REMOVED_SPTE)
> +   If other vcpu froze the entry, restart page fault.
> +3. TLB shootdown
> +
> +   * send IPI to remote vcpus
> +   * TLB flush (local and remote)
> +
> +   For each entry update, TLB shootdown is needed because of the
> +   concurrency.

Concurrency issues?

Also, as I have iterated several times before, you need to add the
documentation to KVM table of contents (index):

---- >8 ----

diff --git a/Documentation/virt/kvm/index.rst b/Documentation/virt/kvm/index.rst
index cdb8b43ce7970a..ff2db9ab428d3c 100644
--- a/Documentation/virt/kvm/index.rst
+++ b/Documentation/virt/kvm/index.rst
@@ -20,3 +20,4 @@ KVM
    review-checklist
 
    intel-tdx
+   tdx-tdp-mmu

Thanks.

-- 
An old man doll... just what I always wanted! - Clara

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