linux-kernel - [PATCH v4 1/2] KVM: guest_memfd: add generic population via write

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Message-ID: <20250828153049.3922-2-kalyazin@amazon.com>
Date: Thu, 28 Aug 2025 15:31:01 +0000
From: "Kalyazin, Nikita" <kalyazin@...zon.co.uk>
To: "pbonzini@...hat.com" <pbonzini@...hat.com>, "shuah@...nel.org"
	<shuah@...nel.org>
CC: "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"michael.day@....com" <michael.day@....com>, "david@...hat.com"
	<david@...hat.com>, "jthoughton@...gle.com" <jthoughton@...gle.com>, "Roy,
 Patrick" <roypat@...zon.co.uk>, "Thomson, Jack" <jackabt@...zon.co.uk>,
	"Manwaring, Derek" <derekmn@...zon.com>, "Cali, Marco"
	<xmarcalx@...zon.co.uk>, "Kalyazin, Nikita" <kalyazin@...zon.co.uk>
Subject: [PATCH v4 1/2] KVM: guest_memfd: add generic population via write

write syscall populates guest_memfd with user-supplied data in a generic
way, ie no vendor-specific preparation is performed.  This is supposed
to be used in non-CoCo setups where guest memory is not
hardware-encrypted.

The following behaviour is implemented:
 - only page-aligned count and offset are allowed
 - if the memory is already allocated, the call will successfully
   populate it
 - if the memory is not allocated, the call will both allocate and
   populate
 - if the memory is already populated, the call will not repopulate it

Signed-off-by: Nikita Kalyazin <kalyazin@...zon.com>
---
 virt/kvm/guest_memfd.c | 64 +++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 63 insertions(+), 1 deletion(-)

diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 08a6bc7d25b6..1f6f85edace0 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -379,7 +379,9 @@ static int kvm_gmem_mmap(struct file *file, struct vm_area_struct *vma)
 }
 
 static struct file_operations kvm_gmem_fops = {
-	.mmap		= kvm_gmem_mmap,
+	.mmap           = kvm_gmem_mmap,
+	.llseek         = default_llseek,
+	.write_iter     = generic_perform_write,
 	.open		= generic_file_open,
 	.release	= kvm_gmem_release,
 	.fallocate	= kvm_gmem_fallocate,
@@ -390,6 +392,63 @@ void kvm_gmem_init(struct module *module)
 	kvm_gmem_fops.owner = module;
 }
 
+static int kvm_kmem_gmem_write_begin(const struct kiocb *kiocb,
+				     struct address_space *mapping,
+				     loff_t pos, unsigned len,
+				     struct folio **foliop,
+				     void **fsdata)
+{
+	struct file *file = kiocb->ki_filp;
+	pgoff_t index = pos >> PAGE_SHIFT;
+	struct folio *folio;
+
+	if (!PAGE_ALIGNED(pos) || len != PAGE_SIZE)
+		return -EINVAL;
+
+	if (pos + len > i_size_read(file_inode(file)))
+		return -EINVAL;
+
+	folio = kvm_gmem_get_folio(file_inode(file), index);
+	if (IS_ERR(folio))
+		return -EFAULT;
+
+	if (WARN_ON_ONCE(folio_test_large(folio))) {
+		folio_unlock(folio);
+		folio_put(folio);
+		return -EFAULT;
+	}
+
+	if (folio_test_uptodate(folio)) {
+		folio_unlock(folio);
+		folio_put(folio);
+		return -ENOSPC;
+	}
+
+	*foliop = folio;
+	return 0;
+}
+
+static int kvm_kmem_gmem_write_end(const struct kiocb *kiocb,
+				   struct address_space *mapping,
+				   loff_t pos, unsigned len, unsigned copied,
+				   struct folio *folio, void *fsdata)
+{
+	int ret;
+
+	if (copied == len) {
+		kvm_gmem_mark_prepared(folio);
+		ret = copied;
+	} else {
+		filemap_remove_folio(folio);
+		ret = 0;
+	}
+
+	folio_unlock(folio);
+	folio_put(folio);
+
+	return ret;
+}
+
 static int kvm_gmem_migrate_folio(struct address_space *mapping,
 				  struct folio *dst, struct folio *src,
 				  enum migrate_mode mode)
@@ -442,6 +501,8 @@ static void kvm_gmem_free_folio(struct folio *folio)
 
 static const struct address_space_operations kvm_gmem_aops = {
 	.dirty_folio = noop_dirty_folio,
+	.write_begin = kvm_kmem_gmem_write_begin,
+	.write_end = kvm_kmem_gmem_write_end,
 	.migrate_folio	= kvm_gmem_migrate_folio,
 	.error_remove_folio = kvm_gmem_error_folio,
 #ifdef CONFIG_HAVE_KVM_ARCH_GMEM_INVALIDATE
@@ -489,6 +550,7 @@ static int __kvm_gmem_create(struct kvm *kvm, loff_t size, u64 flags)
 	}
 
 	file->f_flags |= O_LARGEFILE;
+	file->f_mode |= FMODE_LSEEK | FMODE_PWRITE;
 
 	inode = file->f_inode;
 	WARN_ON(file->f_mapping != inode->i_mapping);
-- 
2.50.1