linux-kernel - [PATCH v4 5/6] KVM: Documentation: Add the missing description for mmu_valid_gen into kvm_mmu

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Message-ID: <20230912184553.1887764-6-mizhang@google.com>
Date:   Tue, 12 Sep 2023 18:45:52 +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 v4 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 4a82fa016833..16d5d6a1c174 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=0).
+  mmu_valid_gen:
+    The MMU generation of this page, used to fast zap of all MMU pages within a
+    VM without blocking vCPUs too long. 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.
-- 
2.42.0.283.g2d96d420d3-goog