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This page documents the known limitations, constraints, and issues with the Intel QAT Engine. Understanding these limitations will help you design and deploy QAT-accelerated applications effectively.

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Process and Threading Limitations

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Forking Restrictions

When forking within an application, it is not valid for a cryptographic operation to be started in the parent process and completed in the child process.
Limitation: Only one level of forking is permitted. If a child process forks again, the Intel QAT OpenSSL Engine will not be available in that forked (grandchild) process. Impact: Applications that use multi-level forking (e.g., some daemon architectures) may need to reinitialize the QAT Engine in each process level or use alternative process models. Workaround: Design applications to use either:
  • Single-level forking only
  • Thread-based concurrency instead of process forking
  • Process pools that don’t fork after initialization

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Thread-Specific USDM Memory

When the QAT driver is configured with --enable-icp-thread-specific-usdm, memory allocated in one thread must be freed only by the thread that allocated it.
Requirements when using thread-specific USDM:
  1. If QAT Engine is the default OpenSSL engine, OPENSSL_init_ssl() must be called from the same thread that calls OPENSSL_cleanup()
  2. Incorrect cleanup can lead to segmentation faults
  3. Memory allocated in a thread is freed automatically when the thread exits, even if not explicitly freed
Example of correct usage: 
void* worker_thread(void* arg) {
    OPENSSL_init_ssl(0, NULL);
    
    // Perform cryptographic operations
    // ...
    
    OPENSSL_cleanup();  // Must be called from same thread
    return NULL;
}

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Platform-Specific Limitations

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FreeBSD Operating System

Event-driven mode of polling operation is not supported in the FreeBSD operating system or in the qatlib RPM.

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FreeBSD qatlib Header Installation

FreeBSD qatlib header files are not installed to the install path due to a known issue in the driver. Headers must be manually copied: 
# Create destination directory from QATlib 23.09 top directory
./install-sh -c -d '/usr/local/include/qat'

# Install header files
/usr/bin/install -c -m 644 \
  quickassist/include/cpa.h \
  quickassist/include/cpa_dev.h \
  quickassist/include/cpa_types.h \
  quickassist/include/lac/cpa_cy_common.h \
  quickassist/include/lac/cpa_cy_dh.h \
  quickassist/include/lac/cpa_cy_drbg.h \
  quickassist/include/lac/cpa_cy_dsa.h \
  quickassist/include/lac/cpa_cy_ecdh.h \
  quickassist/include/lac/cpa_cy_ecdsa.h \
  quickassist/include/lac/cpa_cy_ecsm2.h \
  quickassist/include/lac/cpa_cy_ec.h \
  quickassist/include/lac/cpa_cy_im.h \
  quickassist/include/lac/cpa_cy_key.h \
  quickassist/include/lac/cpa_cy_kpt.h \
  quickassist/include/lac/cpa_cy_ln.h \
  quickassist/include/lac/cpa_cy_nrbg.h \
  quickassist/include/lac/cpa_cy_prime.h \
  quickassist/include/lac/cpa_cy_rsa.h \
  quickassist/include/lac/cpa_cy_sym_dp.h \
  quickassist/include/lac/cpa_cy_sym.h \
  quickassist/include/dc/cpa_dc.h \
  quickassist/include/dc/cpa_dc_dp.h \
  quickassist/include/dc/cpa_dc_chain.h \
  quickassist/lookaside/access_layer/include/icp_sal_poll.h \
  quickassist/lookaside/access_layer/include/icp_sal_user.h \
  quickassist/lookaside/access_layer/include/icp_sal.h \
  quickassist/lookaside/access_layer/include/icp_sal_versions.h \
  quickassist/utilities/libusdm_drv/qae_mem.h \
  /usr/local/include/qat

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BoringSSL Limitations

  • SVM mode is not supported with BoringSSL library
  • Known build issue with latest BoringSSL commits; use IPP Crypto 2021.10
  • Use the specific BoringSSL commit mentioned in Software Requirements
FreeBSD with BoringSSL: Validated functionally with limited performance validation on NGINX QUIC POC.

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Algorithm Limitations

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Cipher Mode Restrictions

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ENCRYPT_THEN_MAC

QAT Engine does not support ENCRYPT_THEN_MAC (default) mode of operation.
Impact: When Encrypt-then-MAC is negotiated for symmetric ciphers like AES-CBC, requests will not be offloaded via QAT_HW and will use OpenSSL software instead. Workaround: Disable ENCRYPT_THEN_MAC programmatically: 
SSL_CTX_set_options(ctx, SSL_OP_NO_ENCRYPT_THEN_MAC);
Disabling ENCRYPT_THEN_MAC has security implications. Evaluate the security risks for your specific use case before disabling this option.

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AES-CCM Cipher Enablement

AES-CCM ciphers are not enabled in OpenSSL by default.
To enable AES-CCM, manually configure via openssl.cnf: 
openssl_conf = cipher_conf

[cipher_conf]
ssl_conf = cipher_sect

[cipher_sect]
system_default = system_cipher_sect

[system_cipher_sect]
Cipherstring = ALL
Ciphersuites = TLS_AES_128_CCM_SHA256:TLS_AES_256_GCM_SHA384:TLS_CHACHA20_POLY1305_SHA256:TLS_AES_128_GCM_SHA256

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AES-CBC-HMAC-SHA Pipeline Feature

AES-CBC-HMAC-SHA chained ciphers do not support the pipeline feature when built with OpenSSL 3.0.
Reason: The corresponding support is not available in OpenSSL 3.0. See OpenSSL#18298.

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AES-CBC-HMAC-SHA Padding

QAT Engine does not process plaintext if the length is not a multiple of AES_BLOCK_SIZE for chained cipher AES-CBC-HMAC-SHA when built with OpenSSL v3 and above.
Impact: No padding will be added to the plaintext as specified in RFC 5652 or RFC 5246.

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Algorithm-Specific Limitations

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HKDF Restrictions

  • HKDF based on SM3 is not supported in QAT_HW
  • HKDF infolen > 80 is not supported due to QAT driver limitation
Behavior: Requests will fall back to OpenSSL software if fallback is enabled; otherwise, failures will occur.

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Key Protection Technology (KPT)

  • Symmetric keys are not protected by Key Protection Technology
  • SVM mode is not supported with KPT mode

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OpenSSL Version Compatibility

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Library Compatibility

QAT Engine built for OpenSSL 3.0 is only compatible with dependent libraries also linked with OpenSSL 3.0 libraries. The same applies for OpenSSL 1.1.1.
Reason: Due to OpenSSL#17112. Impact: All components must use the same OpenSSL major version:
  • QAT Engine/Provider
  • Application (NGINX, HAProxy, etc.)
  • Dependent libraries (crypto_mb, ipsec_mb, etc.)

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Misleading Error Messages

OpenSSL 1.1.1n and OpenSSL 3.0.2 introduced a misleading error message (undefined symbol: EVP_PKEY_get_base_id) during engine load.
Note: This can be safely ignored as it is not a real failure. Fixed in OpenSSL 1.1.1o and OpenSSL 3.0.3.

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Known Functional Issues

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SSH Daemon (sshd) Incompatibility

There is an issue with sshd daemon application when using QAT as the default OpenSSL engine.
Problem: sshd closes file descriptors associated with QAT engine and driver after initializing OpenSSL. Solution: Apply the fix from OpenSSH 8.7+ (commit c9f7bba).

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QAT_SW SM2 in NTLS Mode

Known issue with QAT_SW SM2 in ntls mode since QAT_SW SM2 doesn’t have plain sign and verify operation support in engine.
Workaround: Disable QAT_SW SM2 to work around the issue with ntls. TLS mode works correctly as it uses digestsign and digestverify.

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Software Fallback Issues (OpenSSL 3.0 Engine)

Known issue with software fallback in OpenSSL 3.0 Engine (only) when disabled via co-existence algorithm bitmap for:
  • PRF
  • HKDF
  • SM2
  • SM3
Note: QAT_HW PRF and QAT_HW HKDF are not accelerated in OpenSSL 3.0 engine due to OpenSSL#21622.

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Co-existence Mode Issues

Known issue in co-existence mode with QAT provider on OpenSSL 3.2 and above during QAT_SW offload when QAT_HW modules are not present.

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Performance Limitations

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Multithreading Performance

In some cases, using QAT Engine with OpenSSL at higher thread counts can produce worse performance due to lock contention issues in OpenSSL.
Symptoms:
  • native_queued_spin_lock_slowpath() consuming CPU process idle time
  • Performance degradation with thread counts > 32
Related Issues:
  • Performance bottleneck with locks in engine_table_select() - OpenSSL#18509
  • 3.0 performance degraded due to locking - OpenSSL#20286
References:

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Algorithm-Specific Performance Issues

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ECDSA with QAT_SW

Known performance scaling issue (performance drop with threads > 32) with ECDSA ciphers in QAT Software acceleration using multithread mode in HAProxy.
Note: This issue is not observed when using RSA ciphers or in multi-process mode.

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SM3 Multithread Performance

SM3 is disabled by default due to performance drop observed in multithread scenarios for all cipher suites.
Reason: Additional locks at engine_table_select introduced by engine digest registration in OpenSSL - OpenSSL#18509.

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Co-existence Mode Scaling

In co-existence mode, performance is lower than QAT_SW only when process number >= 64 due to a known issue.

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Platform-Specific Performance Issues

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NGINX Handshake with OpenSSL 3.0

NGINX handshake performance in OpenSSL 3.0 is slightly slower compared to OpenSSL 1.1.1.
Note: The same behavior is observed in OpenSSL software - OpenSSL#21833.

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QAT 2.0 Platforms

Performance scaling is not linear in QAT 2.0 supported platforms for:
  • ECDSA
  • ChaCha-Poly algorithms

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FreeBSD Performance

  • Performance drop with ECDSAP256 in OpenSSL speed tests with FreeBSD 14 in-tree driver
  • Performance drop in QAT Engine with async-nginx on FreeBSD OS with asymmetric and symmetric ciphers

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Planning for Limitations

When designing QAT-accelerated applications:
  1. Review algorithm support: Ensure your required algorithms are supported and accelerated
  2. Test threading models: Validate performance with your specific thread/process count
  3. Plan fallback strategies: Implement graceful degradation when QAT is unavailable
  4. Monitor performance: Establish baselines and monitor for degradation
  5. Stay updated: Track OpenSSL and QAT driver releases for fixes and improvements
Many limitations are actively being addressed in ongoing development. Check the latest release notes for updates and improvements.

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