linux-kernel - [Patch v4 11/18] KVM: x86/mmu: Add documentation of NUMA aware page table capability

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Date:   Mon,  6 Mar 2023 14:41:20 -0800
From:   Vipin Sharma <vipinsh@...gle.com>
To:     seanjc@...gle.com, pbonzini@...hat.com, bgardon@...gle.com,
        dmatlack@...gle.com
Cc:     jmattson@...gle.com, mizhang@...gle.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Vipin Sharma <vipinsh@...gle.com>
Subject: [Patch v4 11/18] KVM: x86/mmu: Add documentation of NUMA aware page
 table capability

Add documentation for KVM_CAP_NUMA_AWARE_PAGE_TABLE capability and
explain why it is needed.

Signed-off-by: Vipin Sharma <vipinsh@...gle.com>
---
 Documentation/virt/kvm/api.rst | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 62de0768d6aa..7e3a1299ca8e 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -7669,6 +7669,35 @@ This capability is aimed to mitigate the threat that malicious VMs can
 cause CPU stuck (due to event windows don't open up) and make the CPU
 unavailable to host or other VMs.
 
+7.34 KVM_CAP_NUMA_AWARE_PAGE_TABLE
+------------------------------
+
+:Architectures: x86
+:Target: VM
+:Returns: 0 on success, -EINVAL if vCPUs are already created.
+
+This capability allows userspace to enable NUMA aware page tables allocations.
+NUMA aware page tables are disabled by default. Once enabled, prior to vCPU
+creation, any page table allocated during the life of a VM will be allocated
+preferably from the NUMA node of the leaf page.
+
+Without this capability, default feature is to use current thread mempolicy and
+allocate page table based on that.
+
+This capability is useful to improve page accesses by a guest. For example, an
+initialization thread which access lots of remote memory and ends up creating
+page tables on local NUMA node, or some service thread allocates memory on
+remote NUMA nodes and later worker/background threads accessing that memory
+will end up accessing remote NUMA node page tables. So, a multi NUMA node
+guest, can with high confidence access local memory faster instead of going
+through remote page tables first.
+
+This capability is also helpful for host to reduce live migration impact when
+splitting huge pages during dirty log operations. If the thread splitting huge
+page is on remote NUMA node it will create page tables on remote node. Even if
+guest is careful in making sure that it only access local memory they will end
+up accessing remote page tables.
+
 8. Other capabilities.
 ======================
 
-- 
2.40.0.rc0.216.gc4246ad0f0-goog