Recently, I came across a blog post by VMware Platform Engineering Architect William Lam, published this February:
https://williamlam.com/2025/02/quick-tip-auditing-esxi-boot-firmware-type.html
Although the blog is titled “Quick Tip – Auditing ESXi boot firmware type” and does not explicitly state that ESXi 9.0 drops legacy BIOS support, its content – particularly the phrase “legacy BIOS firmware, which has been deprecated and will be removed in a future vSphere release” – strongly suggests that from ESXi 9.0 onward only UEFI boot will be supported.
Insights from William Lam’s Blog
In his post, William shared a practical scenario: a customer needed to determine which hosts in their ESXi environment were still using the now-deprecated legacy BIOS boot, and which had adopted the industry-standard UEFI boot. He explained that beginning with vSphere 8.0 Update 2, a new attribute called firmwareType was added to ESXi hardware information. This can be easily retrieved using a simple PowerCLI command, for example:
scssCopyEdit(Get-VMHost).ExtensionData.Hardware.BiosInfo
This command not only displays the firmware type in use on a host but also helps quickly identify those still running on the old legacy BIOS. For hosts that haven’t been updated to 8.0 Update 2, this field may be empty or not appear at all. For urgent cases, William also provided an alternative method using SSH with the vsish command:
swiftCopyEditvsish -e get /hardware/firmwareType
VMware’s Official Deprecation Notice on Legacy BIOS Support
In addition to William’s blog, VMware has issued a detailed deprecation notice regarding legacy BIOS support (Article ID: 313152, last updated October 22, 2024). The official statement, available at:
https://knowledge.broadcom.com/external/article/313152/deprecation-of-legacy-bios-support-in-vs.html
covers several key points:
- Background and Objectives
VMware plans to gradually eliminate support for traditional BIOS (also known as the Compatibility Support Module, or CSM) on server platforms. Starting with vSphere 7.0 and its updates, new servers will no longer be allowed to be certified with legacy BIOS on the VMware Compatibility Guide. However, servers that were using legacy BIOS before certification will continue to receive support temporarily—as long as their vendors maintain legacy BIOS support. - Restrictions on Next-Generation Servers
VMware will not allow next-generation server platforms (such as 3rd Generation Intel® Xeon® Scalable processors and AMD EPYC™ 7003 series processors) to certify legacy BIOS support on older versions (below 7.0). In fact, vSphere 8.0 has already removed legacy BIOS support for all new server platforms, including Intel Cascade Lake and later, as well as AMD Rome and later. - Upgrade Risks and Migration Recommendations
Upgrading a server that was originally certified and successfully running in legacy BIOS mode to a newer version of ESXi might result in boot failures (for example, errors like “insufficient resources”). This is because the new version of ESXi may not boot correctly in legacy BIOS mode. Therefore, unless the server is switched to UEFI mode—or downgraded to an older ESXi version—it may not run properly. For older servers, VMware recommends the following steps to transition from legacy BIOS to UEFI mode:- Switch to UEFI mode in an older ESXi version.
- Reboot the server using that older version.
- Check and resolve any issues with device aliases or functionality (such as devices that do not support UEFI).
- Once stable operation in UEFI mode is confirmed, upgrade to the new version of ESXi.
- Why Choose UEFI?
UEFI offers significant advantages in programmability, scalability, performance, and security, aligning with a “secure by default” philosophy. Future ESXi releases will introduce UEFI-only features, such as:- UEFI Secure Boot: Ensures that the server boots only trusted software.
- Automatic Boot Sequence Updates: Enhances the ESXi installation process.
- Enhanced Hardware Support: Including persistent memory, TPM 2.0, Intel SGX registration, and future support for DPU/SmartNIC technologies.
- Customer Impact
VMware advises customers to consider the potential impacts of transitioning from legacy BIOS to UEFI—such as changes to network boot infrastructures and the benefits of improved security via UEFI Secure Boot. VMware is actively working with its partners to ensure a smooth transition and recommends that customers review the VMware Compatibility Guide for UEFI boot compatibility information.
Emerging Trends with ESXi 9.0
UEFI not only offers clear benefits in terms of boot speed, security, and extensibility, but it is also the direction in which hardware vendors are heading. It seems highly likely that ESXi 9.0 will continue this trend by further limiting—or even completely eliminating—support for legacy BIOS boot. This would help modernize and secure the entire virtualization platform.
William Lam’s blog also touches on the rationale behind this firmware transition, briefly outlining why future ESXi platforms will move away from the traditional BIOS mechanism:
- Enhanced Security: UEFI provides richer security mechanisms to better defend against firmware-level attacks.
- Standardization and Compatibility: As a global industry standard, UEFI is more compatible with the latest hardware technologies.
- Simplified Management and Maintenance: A unified boot mode simplifies system management, automation, and troubleshooting.
Conclusion
While VMware has not yet officially confirmed that ESXi 9.0 will completely drop support for legacy BIOS, both William Lam’s blog and VMware’s deprecation notice – especially the statement “legacy BIOS firmware, which has been deprecated and will be removed in a future vSphere release” – strongly reinforce this trend.
Given VMware’s continuous push toward UEFI and the tightening of BIOS support on new hardware platforms, it is reasonable to speculate that ESXi 9.0 will likely support only UEFI boot and no longer be compatible with legacy BIOS mode. Therefore, users planning to upgrade to ESXi 9.0—especially enterprises still relying on legacy BIOS—should evaluate their servers’ firmware status and prepare for a migration to UEFI mode to ensure future compatibility and system stability.
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