KVM: Don't pre-reserve xarray entries when storing empty/NULL attributesgrafted

Skip the xarray reservation loop when clearing all memory attributes, as
storing NULL only erases the entry and never needs to allocate, so no
reservation (and no cleanup of a failed one) is required in that case.

Suggested-by: Sean Christopherson <seanjc@google.com>
Cc: David Ballesteros <davimaba.v@proton.me>
Signed-off-by: Zeng Chi <zengchi@kylinos.cn>
Link: https://patch.msgid.link/20260921102442.1232375-1-zeng_chi911@163.com
[sean: split to separate patch]
Signed-off-by: Sean Christopherson <seanjc@google.com>

1 files changed, 187 insertions, 0 deletions

diff --git a/Documentation/crypto/api-samples.rst b/Documentation/crypto/api-samples.rst
new file mode 100644
index 000000000..388bb7d7a
--- /dev/null
+++ b/Documentation/crypto/api-samples.rst
@@ -0,0 +1,187 @@
+Code Examples
+=============
+
+Code Example For Symmetric Key Cipher Operation
+-----------------------------------------------
+
+This code encrypts some data with AES-256-XTS.  For sake of example,
+all inputs are random bytes, the encryption is done in-place, and it's
+assumed the code is running in a context where it can sleep.
+
+::
+
+    static int test_skcipher(void)
+    {
+            struct crypto_skcipher *tfm = NULL;
+            struct skcipher_request *req = NULL;
+            u8 *data = NULL;
+            const size_t datasize = 512; /* data size in bytes */
+            struct scatterlist sg;
+            DECLARE_CRYPTO_WAIT(wait);
+            u8 iv[16];  /* AES-256-XTS takes a 16-byte IV */
+            u8 key[64]; /* AES-256-XTS takes a 64-byte key */
+            int err;
+
+            /*
+             * Allocate a tfm (a transformation object) and set the key.
+             *
+             * In real-world use, a tfm and key are typically used for many
+             * encryption/decryption operations.  But in this example, we'll just do a
+             * single encryption operation with it (which is not very efficient).
+             */
+
+            tfm = crypto_alloc_skcipher("xts(aes)", 0, 0);
+            if (IS_ERR(tfm)) {
+                    pr_err("Error allocating xts(aes) handle: %ld\n", PTR_ERR(tfm));
+                    return PTR_ERR(tfm);
+            }
+
+            get_random_bytes(key, sizeof(key));
+            err = crypto_skcipher_setkey(tfm, key, sizeof(key));
+            if (err) {
+                    pr_err("Error setting key: %d\n", err);
+                    goto out;
+            }
+
+            /* Allocate a request object */
+            req = skcipher_request_alloc(tfm, GFP_KERNEL);
+            if (!req) {
+                    err = -ENOMEM;
+                    goto out;
+            }
+
+            /* Prepare the input data */
+            data = kmalloc(datasize, GFP_KERNEL);
+            if (!data) {
+                    err = -ENOMEM;
+                    goto out;
+            }
+            get_random_bytes(data, datasize);
+
+            /* Initialize the IV */
+            get_random_bytes(iv, sizeof(iv));
+
+            /*
+             * Encrypt the data in-place.
+             *
+             * For simplicity, in this example we wait for the request to complete
+             * before proceeding, even if the underlying implementation is asynchronous.
+             *
+             * To decrypt instead of encrypt, just change crypto_skcipher_encrypt() to
+             * crypto_skcipher_decrypt().
+             */
+            sg_init_one(&sg, data, datasize);
+            skcipher_request_set_callback(req, CRYPTO_TFM_REQ_MAY_BACKLOG |
+                                               CRYPTO_TFM_REQ_MAY_SLEEP,
+                                          crypto_req_done, &wait);
+            skcipher_request_set_crypt(req, &sg, &sg, datasize, iv);
+            err = crypto_wait_req(crypto_skcipher_encrypt(req), &wait);
+            if (err) {
+                    pr_err("Error encrypting data: %d\n", err);
+                    goto out;
+            }
+
+            pr_debug("Encryption was successful\n");
+    out:
+            crypto_free_skcipher(tfm);
+            skcipher_request_free(req);
+            kfree(data);
+            return err;
+    }
+
+
+Code Example For Use of Operational State Memory With SHASH
+-----------------------------------------------------------
+
+::
+
+
+    struct sdesc {
+        struct shash_desc shash;
+        char ctx[];
+    };
+
+    static struct sdesc *init_sdesc(struct crypto_shash *alg)
+    {
+        struct sdesc *sdesc;
+        int size;
+
+        size = sizeof(struct shash_desc) + crypto_shash_descsize(alg);
+        sdesc = kmalloc(size, GFP_KERNEL);
+        if (!sdesc)
+            return ERR_PTR(-ENOMEM);
+        sdesc->shash.tfm = alg;
+        return sdesc;
+    }
+
+    static int calc_hash(struct crypto_shash *alg,
+                 const unsigned char *data, unsigned int datalen,
+                 unsigned char *digest)
+    {
+        struct sdesc *sdesc;
+        int ret;
+
+        sdesc = init_sdesc(alg);
+        if (IS_ERR(sdesc)) {
+            pr_info("can't alloc sdesc\n");
+            return PTR_ERR(sdesc);
+        }
+
+        ret = crypto_shash_digest(&sdesc->shash, data, datalen, digest);
+        kfree(sdesc);
+        return ret;
+    }
+
+    static int test_hash(const unsigned char *data, unsigned int datalen,
+                 unsigned char *digest)
+    {
+        struct crypto_shash *alg;
+        char *hash_alg_name = "sha1-padlock-nano";
+        int ret;
+
+        alg = crypto_alloc_shash(hash_alg_name, 0, 0);
+        if (IS_ERR(alg)) {
+                pr_info("can't alloc alg %s\n", hash_alg_name);
+                return PTR_ERR(alg);
+        }
+        ret = calc_hash(alg, data, datalen, digest);
+        crypto_free_shash(alg);
+        return ret;
+    }
+
+
+Code Example For Random Number Generator Usage
+----------------------------------------------
+
+::
+
+
+    static int get_random_numbers(u8 *buf, unsigned int len)
+    {
+        struct crypto_rng *rng = NULL;
+        char *drbg = "stdrng";
+        int ret;
+
+        if (!buf || !len) {
+            pr_debug("No output buffer provided\n");
+            return -EINVAL;
+        }
+
+        rng = crypto_alloc_rng(drbg, 0, 0);
+        if (IS_ERR(rng)) {
+            pr_debug("could not allocate RNG handle for %s\n", drbg);
+            return PTR_ERR(rng);
+        }
+
+        ret = crypto_rng_get_bytes(rng, buf, len);
+        if (ret < 0)
+            pr_debug("generation of random numbers failed\n");
+        else if (ret == 0)
+            pr_debug("RNG returned no data");
+        else
+            pr_debug("RNG returned %d bytes of data\n", ret);
+
+    out:
+        crypto_free_rng(rng);
+        return ret;
+    }