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Re: memory overcomittment with sr-iov device assighment
- To: alex.nlnnfn@xxxxxxxxx
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Tue, 7 Jun 2022 09:11:21 +0200
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- Cc: "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Tue, 07 Jun 2022 07:11:31 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 07.06.2022 05:59, alex.nlnnfn@xxxxxxxxx wrote:
> I looked into Xen documentation and also Xen wiki and I could't find a
> definitive answer if Xen supports memory over-commitment when VMs use SR-IOV
> device assignment (passthrough). Memory over-commitment I mean giving VMs
> more RAM than is available in the host.
>
> I know that ESX doesn't support it, and also QEMU/KVM pins all RAM when a
> device is directly assigned to a VM (VFIO_IOMMU_MAP_DMA ioctl). I have two
> questions:
>
> 1. Does Xen supports memory over commitment when all VMs are using direct
> device assignment e.g., SR-IOV?
What you describe looks to match mem-paging / mem-sharing in Xen. Neither is
compatible with device pass-through (SR-IOV or not is irrelevant), and both
are only in "experimental" state anyway.
> 2. Would you please point me to the code that does the pinning?
That's a KVM concept with no direct equivalent in Xen (largely due to Xen
being a type-1 hypervisor, while KVM is a type-2 one).
Jan
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