PCIe Address Translation Services (ATS) not enabled on Windows Guests running on RHEL KVM

Environment

• Red Hat Enterprise Linux 8 and 9
• Microsoft Windows Guest

Issue

• KVM Virtual Machine is configured with ATS for a device, but Windows Guest is not reporting ATS enabled in Device Manager.

Root Cause

• While ATS is enabled on the hypervisor side, it does not necessarily mean the Guest OS will use it. The Guest OS may decide not to.
• Microsoft Windows only supports ATS for a specific subset of devices, not virtio ones.

Diagnostic Steps

• Virtual Machine has ATS for a device:

<domain>
    <...>
    <devices>
      <...>
      <some device>
        <...>
        <driver iommu='on' ats='on'/>
        <...>
      </some device>
    </devices>
</domain>

• But ATS is not shown in Windows Device Manager -> Device -> Properties -> Details .

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