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[RFC,gmem,v1,2/8] KVM: Use AS_INACCESSIBLE when creating guest_memfd inode

Message ID 20231016115028.996656-3-michael.roth@amd.com (mailing list archive)
State Not Applicable
Delegated to: Herbert Xu
Headers show
Series KVM: gmem hooks/changes needed for x86 (other archs?) | expand

Commit Message

Michael Roth Oct. 16, 2023, 11:50 a.m. UTC 
truncate_inode_pages_range() may attempt to zero pages before truncating
them, and this will occur before arch-specific invalidations can be
triggered via .invalidate_folio/.free_folio hooks via kvm_gmem_aops. For
AMD SEV-SNP this would result in an RMP #PF being generated by the
hardware, which is currently treated as fatal (and even if specifically
allowed for, would not result in anything other than garbage being
written to guest pages due to encryption). On Intel TDX this would also
result in undesirable behavior.

Set the AS_INACCESSIBLE flag to prevent the MM from attempting
unexpected accesses of this sort during operations like truncation.

This may also in some cases yield a decent performance improvement for
guest_memfd userspace implementations that hole-punch ranges immediately
after private->shared conversions via KVM_SET_MEMORY_ATTRIBUTES, since
the current implementation of truncate_inode_pages_range() always ends
up zero'ing an entire 4K range if it is backing by a 2M folio.

Link: https://lore.kernel.org/lkml/ZR9LYhpxTaTk6PJX@google.com/
Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Michael Roth <michael.roth@amd.com>
---
 virt/kvm/guest_memfd.c | 1 +
 1 file changed, 1 insertion(+)
diff mbox series

Patch

diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 9ffce54555ae..f6f1b17a319c 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -398,6 +398,7 @@  static int __kvm_gmem_create(struct kvm *kvm, loff_t size, u64 flags)
 	inode->i_private = (void *)(unsigned long)flags;
 	inode->i_op = &kvm_gmem_iops;
 	inode->i_mapping->a_ops = &kvm_gmem_aops;
+	inode->i_mapping->flags |= AS_INACCESSIBLE;
 	inode->i_mode |= S_IFREG;
 	inode->i_size = size;
 	mapping_set_gfp_mask(inode->i_mapping, GFP_HIGHUSER);