General notes about the transition to PSA

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

docs/4.0-migration-guide/psa-only.md

@@ -0,0 +1,15 @@
+## PSA as the only cryptography API
+
+The PSA API is now the only API for cryptographic primitives.
+
+### Impact on application code
+
+The X.509, PKCS7 and SSL always use PSA for cryptography, with a few exceptions documented in the [PSA limitations](../architecture/psa-migration/psa-limitations.md) document. (These limitations are mostly transparent unless you want to leverage PSA accelerator drivers.) This corresponds to the behavior of Mbed TLS 3.x when `MBEDTLS_USE_PSA_CRYPTO` is enabled. In effect, `MBEDTLS_USE_PSA_CRYPTO` is now always enabled.
+
+`psa_crypto_init()` must be called before performing any cryptographic operation, including indirect requests such as parsing a key or certificate or starting a TLS handshake.
+
+A few functions take different parameters to migrate them to the PSA API. See “[Function prototype changes](#function-prototype-changes)”.
+
+### Impact on the library configuration
+
+Mbed TLS follows the configuration of TF-PSA-Crypto with respect to cryptographic mechanisms. They are now based on `PSA_WANT_xxx` macros instead of legacy configuration macros such as `MBEDTLS_RSA_C`, `MBEDTLS_PKCS1_V15`, etc. The configuration of X.509 and TLS is not directly affected by the configuration. However, applications and middleware that rely on these configuration symbols to know which cryptographic mechanisms to support will need to migrate to `PSA_WANT_xxx` macros. For more information, consult the PSA transition guide in TF-PSA-Crypto.