Deprecation of legacy BIOS support in vSphere

Products

• VMware vSphere ESXi

Issue/Introduction

This article details VMware’s plans to deprecate support for legacy BIOS in server platforms, also known as the Compatibility Support Module (CSM).

 

Environment

VMWare vSphere ESXi

Resolution

Our deprecation of legacy BIOS will be handled as follows:

• As of vSphere 7.0 and all newer releases, VMware will no longer allow new server listings on our VMware Compatibility Guide using legacy BIOS.
• However, servers that are already listed on the VMware Compatibility Guide prior to and have already been certified with legacy BIOS support will continue to be supported, so long as our server partner continues to support legacy BIOS on those servers.
• VMware will no longer allow next generation server platforms to be certified with legacy BIOS on releases older than 7.0
  • 3rd Generation Intel® Xeon® Scalable Processors - Platinum 8300 and Gold 6300/5300 series, codenamed “Ice Lake” or newer
  • AMD EPYC™ 7003 Series Processor codenamed “Milan” or newer
• vSphere 8.0 will drop support for legacy BIOS for new server platforms (Intel Cascade Lake and onwards; AMD Rome and onwards). This will apply to all new server platforms, regardless of the server CPU series.
• If you upgrade a server that was certified and running successfully with legacy BIOS to a newer release of ESXi, it is possible the server will no longer function with that release. For example, some servers may fail to boot with an "Out of resources" message because the newer ESXi release is too large to boot in legacy BIOS mode. Generally, VMware will not provide any fix or workaround for such issues besides either switching the server to UEFI (see "BIOS switch" section below) or downgrading back to the old release.

Motivation

UEFI provides several advantages over legacy BIOS and aligns with VMware goals for being “secure by default”. UEFI in general has better programmability, greater scalability, higher performance and higher security compared to legacy BIOS. Here is a partial list of ESXi features that require UEFI:

• UEFI Secure Boot, a security standard that helps ensure that the server boots using only software that is trusted by the server manufacturer.
• Automatic update of the system boot order during ESXi installation.
• Persistent memory
• TPM 2.0
• Intel SGX Registration
• Upcoming support for DPU/SmartNIC

As time goes on, ESXi is likely to add more and more features that function only with UEFI, not legacy BIOS.

Customer Impact

VMware has supported UEFI boot for about 8 years and can assure customers that our support is robust. However, customers are advised to consider the impact of running UEFI boot mode may have in their environment. Some examples include the transition to new network-based boot infrastructure or improved security through UEFI Secure Boot support in vSphere.

VMware is working with our partner ecosystem to ensure that the deprecation of legacy BIOS is a smooth transition. Check the VMware Compatibility Guide under Systems/Servers for UEFI boot compatibility.

BIOS switch between legacy and UEFI modes on older servers

If you need to switch a machine that has ESXi installed in legacy BIOS mode to UEFI mode when upgrading ESXi, here is the recommended procedure.

1. Switch from Legacy BIOS to UEFI mode while still in the older release.
2. Reboot with the older release.
3. Resolve device issues that could result from switching to UEFI mode:

a. Device alias name changes. Please refer to How VMware ESXi determines the order in which names are assigned to devices and After upgrading the HBA card driver, storage adapter names change for some of the FC vmhbas for resolution.

b. Device functional issues if UEFI is not supported by the device. Reach out to your hardware vendor to resolve any issues with device functionality in UEFI mode.

4. If and only if the system is functioning as expected in UEFI mode, then proceed with upgrading ESXi to the new release.

Future considerations for vSphere support of legacy BIOS
In a future version of vSphere, VMware will completely deprecate and remove support for legacy BIOS, and hence strongly recommends that customers start evaluating the UEFI boot mode as default in their environment.

Additional Information

Although we are actively working on deprecation of legacy BIOS support for the host ESXi hypervisor itself, recommending UEFI boot for all new server platforms with vSphere 8.0 and later releases. BUT we provide full support for legacy BIOS for guest OSs, although UEFI is the recommended and default firmware for modern systems.

Virtual Machine Boot (Guest OS):

• Legacy BIOS: Still fully supported in all current versions (including vSphere 8 and VCF 9). It is often the default for older Guest OS templates (e.g., Windows Server 2012, RHEL 6).

• UEFI: The standard for modern Guest OSs (Windows 10/11, Server 2019/2022).