topic Regardless of how you in Intel® ISA Extensions

IA-32e 64-bit and compatibility mode

Ravi_K_1 — Tue, 15 Sep 2015 12:12:59 GMT

Hi,

From Intel developer manual I see

IA-32e mode allows software to operate in one of two sub-modes:

64-bit mode supports 64-bit OS and 64-bit applications
Compatibility mode allows most legacy software to run; it co-exists with 64-bit applications under a 64-bit OS

I have a quad-core with HT enabled i.e. 8 logical cores

With KVM I want to run 32 bit and 64 bit VMs on a 64 bit Host OS. I want to allocate resources as follows

VM-1 64 bits 2-VCPUs (1 physical-core with HT enabled i.e. 2 logical cores/threads)
VM-2 32 bits 6-VCPUs (3 physical-cores with HT enabled i.e. 6 logical cores/threads)

If VM-1 2-VCPUs are assigned per-thread i.e. they can end up on 2 physical cores shared with VM-2. If this happens cpu switching between compatibility and 64-bit mode can impact performance?

If I make sure VM-1 2-VCPUs are assigned to 1 physical-core and its enabled for 64-bit mode and VM-2 6-VCPUs are assigned to 3-physical cores and enabled for compatibility mode then we can avoid mode switching?

In general will there be performance impact on switching between 2 CPU modes? What additional things I need to consider to run both 32-bit and 64-bit VMs without any performance impact?

Thanks,

Ravi

Anyone has some inputs to

Ravi_K_1 — Sat, 26 Sep 2015 02:56:55 GMT

Anyone has some inputs to share?

Thanks.

Regardless of how you

andysem — Sat, 26 Sep 2015 10:59:07 GMT

Regardless of how you distribute the VMs between CPU cores, CPU will switch between 32 and 64-bit mode - at the very least to run the host code. I can't tell how expensive the switch is but IMO it is negligible compared to the costs of virtualization itself. If you want performance, avoid VMs of any kind.

Thanks andysem, in our case

Ravi_K_1 — Wed, 30 Sep 2015 19:26:48 GMT

Thanks andysem, in our case host is minimally involved because of mostly user-space code. Are there tools from Intel which can be used to measure a switch between 32 and 64bit mode?