linux-kernel - [PATCH 0/4] KVM: x86/mmu: pte_list

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Message-Id: <20220624232735.3090056-1-seanjc@google.com>
Date:   Fri, 24 Jun 2022 23:27:31 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Peter Xu <peterx@...hat.com>
Subject: [PATCH 0/4] KVM: x86/mmu: pte_list_desc fix and cleanups

Reviewing the eager page splitting code made me realize that burning 14
rmap entries for nested TDP MMUs is extremely wasteful due to the per-vCPU
caches allocating 40 entries by default.  For nested TDP, aliasing L2 gfns
to L1 gfns is quite rare and is not performance critical (it's exclusively
pre-boot behavior for sane setups).

Patch 1 fixes a bug where pte_list_desc is not correctly aligned nor sized
on 32-bit kernels.  The primary motivation for the fix is to be able to add
a compile-time assertion on the size being a multiple of the cache line
size, I doubt anyone cares about the performance/memory impact.

Patch 2 tweaks MMU setup to support a dynamic pte_list_desc size.

Patch 3 reduces the number of sptes per pte_list_desc to 2 for nested TDP
MMUs, i.e. allocates the bare minimum to prioritize the memory footprint
over performance for sane setups.

Patch 4 fills the pte_list_desc cache if and only if rmaps are in use,
i.e. doesn't allocate pte_list_desc when using the TDP MMU until nested
TDP is used.

Sean Christopherson (4):
  KVM: x86/mmu: Track the number entries in a pte_list_desc with a ulong
  KVM: x86/mmu: Defer "full" MMU setup until after vendor
    hardware_setup()
  KVM: x86/mmu: Shrink pte_list_desc size when KVM is using TDP
  KVM: x86/mmu: Topup pte_list_desc cache iff VM is using rmaps

 arch/x86/include/asm/kvm_host.h |  5 ++-
 arch/x86/kvm/mmu/mmu.c          | 78 +++++++++++++++++++++++----------
 arch/x86/kvm/x86.c              | 17 ++++---
 3 files changed, 70 insertions(+), 30 deletions(-)

base-commit: 4b88b1a518b337de1252b8180519ca4c00015c9e
-- 
2.37.0.rc0.161.g10f37bed90-goog