login vsi company logo login vsi company logo 250x40

Testing L1TF patches: Virtual Desktops on VMware ESX

Last week Intel announced 3 new severe vulnerabilities in their processors allowing unauthorized access to the data in the L1-cache. They have been named L1 Terminal Fault or in short L1TF. By now I assume most IT-admins are aware of this, but the performance impact remains a mystery. That’s why we took it upon us to get you this information as soon as possible. Please note that these are initial findings. As we are doing more research and get more results we will constantly publish new updates.

CVE-2018-3615 L1 Terminal Fault-SGX
CVE-2018-3620 L1 Terminal Fault-OS/ SMM
CVE-2018-3646 L1 Terminal Fault-VMM

In this article I’m going to focus on the third variant (3646) as I expect it to have the biggest impact on the scalability and performance of Virtual Desktop environments such as VMware Horizon View and Citrix XenApp / XenDesktop. For those of you on AMD CPU’s there is good news, as they do not seem to be affected.

How can this leak be exploited? Simply said a malicious virtual machine (VM) running on a certain CPU core can access privileged information of another VM that is on the same CPU core at the same time and read its L1 Data Cache. This is possible because Intel processors share physically addressed L1 Data Cache across both logical processors of a Hyperthreading enabled core. When patching is not an option a quick way to mitigate this could be to disable hyperthreading although that might have a significant impact on performance or cause capacity issues and is therefore discouraged by VMware:“Disabling Intel Hyperthreading in firmware/BIOS (or by using VMkernel.Boot.Hyperthreading) after applying vSphere updates and patches is not recommended and precludes potential vSphere scheduler enhancements and mitigations that will allow the use of both logical processors. As such, disablement of hyperthreading to mitigate the Concurrent-context attack vector will introduce unnecessary operational overhead as hyperthreading may need to be re-enabled in the future.”

I have conducted my tests on VMware ESXi 6.5.0 Update 2 (Build 9298722) and my preliminary research is focused on the VMkernel.Boot.HyperthreadingMitigation setting that restricts the simultaneous use of logical processors from the same hyperthreaded core as necessary to mitigate a security vulnerability. This is the most reliable way to prevent exploits as all virtual machines are considered untrusted siblings. This level of security is needed e.g. in cloud desktop environments and high secure environments as financial institutions or hospitals. Meanwhile my college Tom is performing these tests on Citrix XenServer, but more on that later.


According to VMware it is safe to patch vCenter and/or the ESXi hosts as the mitigation is disabled by default. This is a great way to get ready for the next step: researching capacity issues. Naturally I am using the industry standard load testing solution Login VSI to simulate users on my environment. To start I have installed the patch but have left VMkernel.Boot.HyperthreadingMitigation to its default setting: False. A friendly message notifies me of this setting after the update is complete.

L1TF 2

As my previous tests where on Server 2016 I decided to start measuring the impact on that platform as it would save some setup time. While testing bare RDSH machines it is expected that the relative impact will be similar on Citrix XenApp and Horizon shared session hosts but make sure to validate this in your own environments.

I have deployed 6 Windows server 2016 machines with 4vCPU’s and 55GB of memory resulting in an environment that could run 196 users before VSImax was hit. This is the maximum number of users that can work on an environment before performance becomes a bottleneck. Interestingly enabling HTMitigation did not impact performance too much, at first this had us wondering but discussions with performance experts quickly led us to conclude that the number of VM’s and vCPU’s simply allowed the hypervisor to work out a scenario where core’s where not shared.

Windows Server 2016 (Default) 196 700 1677
Windows Server 2016 with HTM enabled 193 694 1678

L1TF 3

So, we changed the configuration, now running with 8 Server 2016 machines each with 32GB of memory and 6vCPU’s. This slightly lowered VSImax to 186 users.

Windows Server 2016 (Default) 196 700 1677
Windows Server 2016 with HTM enabled 186 694 1532

L1TF 4

L1TF 5

All right, knowing this it was time to step up the game and switch to Windows 10. I started out with a fresh copy of build 17134.1 (1803) with no further Windows updates and gave it a spin. In our lab we deployed a 180 VM’s al equipped with 2GB of memory and 2 vCPU’s and kicked of a test. As you can see the VSImax drops approximately 20%.

Windows 10 140 1023 1977
Windows 10 with HTMitigation enabled 110 1004 1963

L1TF 6

As you can see the performance/density? hit is significant, there are however nuances: different operating systems, newer (or older) CPU’s and of course the applications and infrastructure in your own environment will be of influence on the exact impact. In addition: It also seems that the impact of the L1TF patch depends heavily on your configuration. When using RDS machines in an efficient way (when hyperthreading is fully utilized) the patch has minimal impact. However, we do see a bigger impact when you do utilize the Hyperthreading tech i.e. VDI.

Please note that these are the first results - many thanks to Jasper Geelen for the help and assistance - and we still have many questions remaining so updates are to be expected. If you like to get more info feel free to reach out, or if you’d like to test your own environment: Download your trial of Login VSI today.



Start using Login VSI today

Our industry-standard software is built to help you avoid problems, lower costs and improve performance. Request a Quote or get your (free!) Trial below, and benefit from our award-winning services.


Request a Quote  Request a Trial




About the author

Mark Plettenberg (@markplettenberg) is a product manager of Login VSI and has played a critical role in the development and growth of Login VSI. Ask Mark about motorcycle mechanics and breaking/repairing anything and everything that has a power plug.

Tags: News, Login VSI

Popular Blogs


The VDI Performance Summit - Virtual Conference and Expo

Visit the VDI Performance Summit to gain knowledge and experience about performance and tuning VDI, improving End-User Experience and IT service. Join us at the ONLY virtual event 100% dedicated to VDI performance and tuning | May 2, 2019 This 1-day event offers key-notes presented by the best VDI performance experts in the world, technical and business oriented breakout sessions, the possibility to chat with experts directly to discuss your own situation, and a virtual exhibit hall featuring… Continue Reading

Scalability testing Parallels Remote Application Server with Login VSI

Recently I went to VMworld in Barcelona where Login VSI had a booth on the expo... While I can’t remember exactly how many conversations I had - there had been so many that I lost my voice on day one. What was new this year is that quite a few people asked if our software is compatible with the solutions from Parallels specifically their Remote Application Server (RAS) (Datasheet). Continue Reading
Login VSI Blog Article - Microsoft Windows 10 Default FTA Associations - Teaser Image

Windows 10 Default File Type Associations and Login VSI

When Login VSI 4.1 was released, the majority of desktops were running Windows 7 and life was easy. We’d set the default filetype for an application and it would simply work. The default and industry standard workloads in Login VSI include launching and using Adobe Reader as part of the virtual user simulation. Because Login VSI doesn’t always know which version of Adobe Reader is installed, or where it’s installed, the workload relies on the file type association (FTA) for .pdf documents to be… Continue Reading
Login VSI - Press Release - IGEL - Login VSI Partner to Optimize End User Computing Experience Image

[Press Release] IGEL Partners with Login VSI to Optimize the End User Computing Experience

Login PI enables organizations to better protect the performance and availability of their IGEL OS-powered virtual desktop environments San Francisco, USA, Feb. 6, 2019: IGEL, a world leader in software-defined endpoint optimization and control solutions for the secure enterprise, today announced that it is partnering with Login VSI, provider of software solutions to test and actively monitor the performance and availability of virtual desktop environments, including VDI and… Continue Reading
Login VSI - Blog - Login PI Blog Teaser Image - Windows Virtual Desktop: How To Monitor User Experience With Login PI

Windows Virtual Desktop – How to Monitor User Experience?

Microsoft has just announced the public preview of their new Windows Virtual Desktop (WVD) offering at Microsoft Ignite on Tour in Amsterdam today. For those of you who’ve not followed the rumors or the private beta, here’s the outline... Continue Reading
Investigating Online Application Performance with Login PI

Investigating Online Application Performance with Login PI

As many companies do, we use a CRM system. Recently, I have been getting complaints about our cloud CRM system, Microsoft Dynamics, being slow. I tried to investigate this by shadowing one of our users to see what was wrong. As expected, everything was fast. 15 minutes later, the same user reported slowness again. How could I investigate this without bothering the users? Continue Reading
Cookie Settings