Re: [RFC PATCH v2 28/69] KVM: Add per-VM flag to mark read-only memory as unsupported

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On 7/6/21 9:03 AM, Paolo Bonzini wrote:
On 03/07/21 00:04, isaku.yamahata@xxxxxxxxx wrote:
From: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>

Add a flag for TDX to flag RO memory as unsupported and propagate it to
KVM_MEM_READONLY to allow reporting RO memory as unsupported on a per-VM
basis.  TDX1 doesn't expose permission bits to the VMM in the SEPT
tables, i.e. doesn't support read-only private memory.

Signed-off-by: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
Co-developed-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
---
  arch/x86/kvm/x86.c       | 4 +++-
  include/linux/kvm_host.h | 4 ++++
  virt/kvm/kvm_main.c      | 8 +++++---
  3 files changed, 12 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index cd9407982366..87212d7563ae 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3897,7 +3897,6 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
      case KVM_CAP_ASYNC_PF_INT:
      case KVM_CAP_GET_TSC_KHZ:
      case KVM_CAP_KVMCLOCK_CTRL:
-    case KVM_CAP_READONLY_MEM:
      case KVM_CAP_HYPERV_TIME:
      case KVM_CAP_IOAPIC_POLARITY_IGNORED:
      case KVM_CAP_TSC_DEADLINE_TIMER:
@@ -4009,6 +4008,9 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
          if (static_call(kvm_x86_is_vm_type_supported)(KVM_X86_TDX_VM))
              r |= BIT(KVM_X86_TDX_VM);
          break;
+    case KVM_CAP_READONLY_MEM:
+        r = kvm && kvm->readonly_mem_unsupported ? 0 : 1;
+        break;
      default:
          break;
      }
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index ddd4d0f68cdf..7ee7104b4b59 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -597,6 +597,10 @@ struct kvm {
      unsigned int max_halt_poll_ns;
      u32 dirty_ring_size;
+#ifdef __KVM_HAVE_READONLY_MEM
+    bool readonly_mem_unsupported;
+#endif
+
      bool vm_bugged;
  };
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 52d40ea75749..63d0c2833913 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1258,12 +1258,14 @@ static void update_memslots(struct kvm_memslots *slots,
      }
  }
-static int check_memory_region_flags(const struct kvm_userspace_memory_region *mem)
+static int check_memory_region_flags(struct kvm *kvm,
+                     const struct kvm_userspace_memory_region *mem)
  {
      u32 valid_flags = KVM_MEM_LOG_DIRTY_PAGES;
  #ifdef __KVM_HAVE_READONLY_MEM
-    valid_flags |= KVM_MEM_READONLY;
+    if (!kvm->readonly_mem_unsupported)
+        valid_flags |= KVM_MEM_READONLY;
  #endif
      if (mem->flags & ~valid_flags)
@@ -1436,7 +1438,7 @@ int __kvm_set_memory_region(struct kvm *kvm,
      int as_id, id;
      int r;
-    r = check_memory_region_flags(mem);
+    r = check_memory_region_flags(kvm, mem);
      if (r)
          return r;


For all these flags, which of these limitations will be common to SEV-ES and SEV-SNP (ExtINT injection, MCE injection, changing TSC, read-only memory, dirty logging)?  Would it make sense to use vm_type instead of all of them?  I guess this also guides the choice of whether to use a single vm-type for TDX and SEV-SNP or two.  Probably two is better, and there can be static inline bool functions to derive the support flags from the vm-type.


The SEV-ES does not need any of these flags. However, with SEV-SNP, we may able use the ExtINT injection.

-Brijesh




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