Host BIOS Updates and VM's

[nabeelsayegh]

Got what may be a dumb question, but maybe not.  We are doing a point release of BIOS/Firmware updates to our Dell ESXi 5.1 U1 hosts.   We have several clusters and after hours we are taking one host out at a time to perform the said updates. The process as you would imagine involved vMotioning all the VM's off the host onto others in the cluster and them move them back when the BIOS updates are complete.  A question came up today regarding if the VM's get the BIOS update/benefits automatically or if they need to be power cycled in order to 'take advantage of' the new BIOS function/feature/code.  I did not have a 100% answer for this, but it is my understanding that so long as it is the same make/model of CPU's, you don't need to power cycle the VM's.  However, if you are updating to a newer model of CPU (but not the make), then you would need to power cycle VM's in order for them to get the new stepping code. Can someone point me to some documentation about when VM's reboots/power cycles are required/recommended based on certain hardware updates.  I would expand this beyond BIOS updates to also updating things like:

RAID Controllers

HDD/SSD drives

FC HBA's

ESX hardware Drivers

Network Adapters

Misc Function adapters (Like FusionIO or vfCache...or whatever they are calling it these days.)

[zXi_Gamer]

A question came up today regarding if the VM's get the BIOS update/benefits automatically or if they need to be power cycled in order to 'take advantage of' the new BIOS function/feature/code

Well, it is a questionable call, but let me try to explain in my view.

With a BIOS change, the vendor can give anything ranging from small bug fixes to a change in microcode leading to disabling of a feature or change of a feature say VT-d, flex, CPU power operations.

These changes will be in turned used by the virtual machines, since in VM world, we go for hardware assisted virtualization when possible, ie HW MMU.

Along with any feature of the processor that supports virtualization.

I do remember jmattson's blog about the flex priority, MMUs and its implication on virtual machines.

So, yes any BIOS change will do inturn have impact on the virtual machines.

[nabeelsayegh]

So the million dollar question is, do the enhancements in BIOS updates get realized in the VM in real time or on a power cycle of the VM?

[admin]

That depends on the BIOS update, the guest OS, and the application.

CPU bug-fixes contained in the BIOS update will be available to the guest immediately.  Typically, this is sufficient.

However, if the guest OS checked the microcode patch level at boot and it disabled some feature due to a known CPU bug, the guest OS is unlikely to check for a microcode update until it is booted again.

On the other hand, if an application has disabled some CPU feature due to a known CPU bug, it is quite likely that restarting the application will be sufficient for it to observe the new microcode patch level and allow the feature to be used (without rebooting the guest OS).