linux-kernel - [PATCH v3 5/6] KVM: Documentation: Add the missing description for mmu_valid_gen into kvm_mmu

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Message-ID: <20230801002127.534020-6-mizhang@google.com>
Date:   Tue,  1 Aug 2023 00:21:25 +0000
From:   Mingwei Zhang <mizhang@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     kvm@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, Mingwei Zhang <mizhang@...gle.com>,
        Kai Huang <kai.huang@...el.com>,
        Jim Mattson <jmattson@...gle.com>,
        David Matlack <dmatlack@...gle.com>,
        Ben Gardon <bgardon@...gle.com>, Xu Yilun <yilun.xu@...el.com>,
        Zhi Wang <zhi.wang.linux@...il.com>,
        Randy Dunlap <rdunlap@...radead.org>
Subject: [PATCH v3 5/6] KVM: Documentation: Add the missing description for
 mmu_valid_gen into kvm_mmu_page

Add the description for mmu_valid_gen into kvm_mmu_page description.
mmu_valid_gen is used in shadow MMU for fast zapping. Update the doc to
reflect that.

Signed-off-by: Mingwei Zhang <mizhang@...gle.com>
Reviewed-by: Kai Huang <kai.huang@...el.com>
---
 Documentation/virt/kvm/x86/mmu.rst | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/Documentation/virt/kvm/x86/mmu.rst b/Documentation/virt/kvm/x86/mmu.rst
index 40daf8beb9b1..581e53fa00a2 100644
--- a/Documentation/virt/kvm/x86/mmu.rst
+++ b/Documentation/virt/kvm/x86/mmu.rst
@@ -208,6 +208,16 @@ Shadow pages contain the following information:
     The page is not backed by a guest page table, but its first entry
     points to one.  This is set if NPT uses 5-level page tables (host
     CR4.LA57=1) and is shadowing L1's 4-level NPT (L1 CR4.LA57=1).
+  mmu_valid_gen:
+    The MMU generation of this page, used to fast zap of all MMU pages within a
+    VM without blocking vCPUs. Specifically, KVM updates the per-VM valid MMU
+    generation which causes the mismatch of mmu_valid_gen for each mmu page.
+    This makes all existing MMU pages obsolete. Obsolete pages can't be used.
+    Therefore, vCPUs must load a new, valid root before re-entering the guest.
+    The MMU generation is only ever '0' or '1'.  Note, the TDP MMU doesn't use
+    this field as non-root TDP MMU pages are reachable only from their owning
+    root.  Thus it suffices for TDP MMU to use role.invalid in root pages to
+    invalidate all MMU pages.
   gfn:
     Either the guest page table containing the translations shadowed by this
     page, or the base page frame for linear translations.  See role.direct.
-- 
2.41.0.585.gd2178a4bd4-goog