Re: [PATCH 02/21] KVM: Allow page-sized MMU caches to be initialized with custom 64-bit values

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On 3/5/2024 2:55 PM, Binbin Wu wrote:


On 2/28/2024 7:20 AM, Paolo Bonzini wrote:
From: Sean Christopherson <seanjc@xxxxxxxxxx>

Add support to MMU caches for initializing a page with a custom 64-bit
value, e.g. to pre-fill an entire page table with non-zero PTE values.
The functionality will be used by x86 to support Intel's TDX, which needs
to set bit 63 in all non-present PTEs in order to prevent !PRESENT page
faults from getting reflected into the guest (Intel's EPT Violation #VE
architecture made the less than brilliant decision of having the per-PTE
behavior be opt-out instead of opt-in).

Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
Signed-off-by: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
Message-Id: <5919f685f109a1b0ebc6bd8fc4536ee94bcc172d.1705965635.git.isaku.yamahata@xxxxxxxxx>
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
---
  include/linux/kvm_types.h |  1 +
  virt/kvm/kvm_main.c       | 16 ++++++++++++++--
  2 files changed, 15 insertions(+), 2 deletions(-)

Reviewed-by: Binbin Wu <binbin.wu@xxxxxxxxxxxxxxx>


diff --git a/include/linux/kvm_types.h b/include/linux/kvm_types.h
index d93f6522b2c3..827ecc0b7e10 100644
--- a/include/linux/kvm_types.h
+++ b/include/linux/kvm_types.h
@@ -86,6 +86,7 @@ struct gfn_to_pfn_cache {
  struct kvm_mmu_memory_cache {
      gfp_t gfp_zero;
      gfp_t gfp_custom;
+    u64 init_value;
      struct kmem_cache *kmem_cache;
      int capacity;
      int nobjs;
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 9c99c9373a3e..c9828feb7a1c 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -401,12 +401,17 @@ static void kvm_flush_shadow_all(struct kvm *kvm)
  static inline void *mmu_memory_cache_alloc_obj(struct kvm_mmu_memory_cache *mc,
                             gfp_t gfp_flags)
  {
+    void *page;
+
      gfp_flags |= mc->gfp_zero;
        if (mc->kmem_cache)
          return kmem_cache_alloc(mc->kmem_cache, gfp_flags);
-    else
-        return (void *)__get_free_page(gfp_flags);
+
+    page = (void *)__get_free_page(gfp_flags);
+    if (page && mc->init_value)
+        memset64(page, mc->init_value, PAGE_SIZE / sizeof(mc->init_value));

Do we need a static_assert() to make sure mc->init_value is 64bit?

+    return page;
  }
    int __kvm_mmu_topup_memory_cache(struct kvm_mmu_memory_cache *mc, int capacity, int min) @@ -421,6 +426,13 @@ int __kvm_mmu_topup_memory_cache(struct kvm_mmu_memory_cache *mc, int capacity,
          if (WARN_ON_ONCE(!capacity))
              return -EIO;
  +        /*
+         * Custom init values can be used only for page allocations,
+         * and obviously conflict with __GFP_ZERO.
+         */
+        if (WARN_ON_ONCE(mc->init_value && (mc->kmem_cache || mc->gfp_zero)))
+            return -EIO;
+
          mc->objects = kvmalloc_array(capacity, sizeof(void *), gfp);
          if (!mc->objects)
              return -ENOMEM;







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