[SOLVED] Help about cpu type

Use host for max performance, but only if you have a singlehost. If you have a cluster, make sure all hosts have same cpu, otherwise livemigration might not work.
 
jsterr said:
Use host for max performance, but only if you have a singlehost. If you have a cluster, make sure all hosts have same cpu, otherwise livemigration might not work.
Click to expand...

Sorry I forgot to mention that I don't want to use host but keep the processor inside of machine "generic", but with the right features.

Both my cpu supports AES so the default x86-64v2 AES should be ok, but maybe I can set the v3 or the v4.. but how can I check if the instructions of the v3 and v4 are supported by the xeons I have?
 
Hi,
crc-error-79 said:
Hello, I have a proxmox server with a Xeon E3-1265L v3 (link intel ark) and another machine with a Xeon E3-1240 v6 (link intel ark), how can I choose the right cpu type?
Click to expand...
if the nodes are in a cluster, you need a CPU type both hosts support for live migration. Otherwise you can just select the most fitting model from the list.

crc-error-79 said:
I checked on the documentation but the page with the list is not available..

https://pve.proxmox.com/wiki/Qemu/KVM_Virtual_Machines#qm_virtual_machines_settings
https://pve.proxmox.com/wiki/Introduction#chapter_qm_vcpu_list
Click to expand...
The wiki page is auto-generated from the documentation and it seems something went wrong with the link during conversion. The list is available here: https://pve.proxmox.com/pve-docs/pve-admin-guide.html#chapter_qm_vcpu_list

crc-error-79 said:
Sorry I forgot to mention that I don't want to use host but keep the processor inside of machine "generic", but with the right features.

Both my cpu supports AES so the default x86-64v2 AES should be ok, but maybe I can set the v3 or the v4.. but how can I check if the instructions of the v3 and v4 are supported by the xeons I have?
Click to expand...
You need >=Broadwell for x86-64-v3 and >=Skylake for x86-64-v4, so I think your Haswell CPU is too old for either of them. QEMU should also tell you that features are not supported by the host CPU and refuse to start if you select and incompatible CPU model.
 
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@DJB-WSM Interesting, that's a different list than Proxmox, who says:
"x86-64-v3: Compatible with Intel CPU >= Broadwell, ..."

Despite the above, x86-64-v3 works on a Haswell here, to start up and migrate a VM back and forth. Haswell-noTSX works also.

Interestingly "Haswell" fails with:
kvm: warning: host doesn't support requested feature: CPUID.07H:EBX.hle [bit 4]
kvm: warning: host doesn't support requested feature: CPUID.07H:EBX.rtm [bit 11]

Haswell-noTSX-IBRS fails with:
kvm: warning: host doesn't support requested feature: CPUID.07H:EDX.spec-ctrl [bit 26]

Haswell-IBRS fails with all 3 of the above errors.

The Proxmox list has "Haswell-IBRS (v3)" and this particular CPU is a Xeon E5-2650 v3. Is that not the same "v3" then?

Thanks.
 
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Follow up, I changed two VMs' CPUs....an Alma 8 server has basically the same CPU usage, maybe .1% higher. An Alma 9 server that should be completely idle (it has some Docker containers, no one's using it) is about .5% higher CPU (3.5% vs 2.96%).

I'm not totally surprised, given other threads like:
I

Thread 'CPU types, word of caution'

Hi everyone,

This is just in case someone stumbles upon the same error while troubleshooting.
I am not asking for help, though I’m open to explanations if anyone has one.

Roughly two years ago, I created a new Proxmox host with VMs.
Two weeks ago, I read about CPU types—something I had never looked into before—and realized I had been using the default x86-64-v2.
Since I don’t run a cluster, I thought it was time to switch to the host CPU type to take advantage of that sweet ZFS speed.

Unfortunately, setting the CPU type to host made some Windows 11 VMs extremely...
D

Thread 'CPU type `host` is significantly SLOWER than `x86-64-v2-AES`'

I am shocked to discover what seems absolutely counter-intuitive to me, but maybe someone can explain me?

I have a stock Win11 24H2 and I initially set it up with x86-64-v2-AES and it was zipping along as one would expect.

1734300836343.png

Then I realized there is a 'host' CPU type and since I have a 128 GB 72 Core Dual Xeon (Lenovo Thinkstation P710) Proxmox server, I figured I'd just allocate directly as "native" HAS to be better/faster than emulated (anything) right? Wrong.

What I didn't expect is that the VM is barely usable!? The CPUs ALL peg at 100% and eventually...
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K

Thread 'The Reasons for poor performance of Windows when the CPU type is host'

Hey guys, I did some experiments recently and I think I finally found out why Windows performs poorly when the CPU type is host. You can check the complete experiment process and conclusion in my blog (Chinese, use google translate!)

In short, the experiment finally found that the reason was that the two flags md_clear and flush_l1d caused performance problems. They would activate the CPU vulnerability mitigation measures of Windows, which would cause a significant increase in memory read latency, thus causing Windows to freeze.

The two flags md_clear and flush_l1d are not passed...
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thats because on windows 11 / windows server 2025 vbs (virtualization based security) is enabled when you install it on real hardware.

if you virtualize it and use "host" as cpu type (and assuming the cpu is a newer model) its enabled too but then you have nested virtualization and that hurts performance.

by changing the cpu type to "x86-64-v2" vbs inside windows vm's gets disabled (cpu instructions for vbs support missing) and the performance gets a boost at the cost of some security.
 
I'm aware of that but does AlmaLinux 9 do that also?

I'm aware we're digressing off the thread a but but one more interesting point, CPU increased again after changing it back to "v2", check the flat spots:
1744475048178.png

¯\_(ツ)_/¯ I'm not worried about a tiny amount, I just think it's odd. Guess we should never restart this VM, ha.

Edit: oops, seems it was still on v3. [After] a few minutes, the v2 CPU is still about 3.5% not 2.97%. Oh well.
 
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