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Re: Limitations for Running Xen on KVM Arm64
- To: "haseeb.ashraf@xxxxxxxxxxx" <haseeb.ashraf@xxxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Demi Marie Obenour <demiobenour@xxxxxxxxx>
- Date: Fri, 31 Oct 2025 22:04:43 -0400
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- Delivery-date: Sat, 01 Nov 2025 02:05:11 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 10/30/25 02:12, haseeb.ashraf@xxxxxxxxxxx wrote:
> Hello Xen development community,
>
> I wanted to discuss the limitations that I have faced while running Xen on
> KVM on Arm64 machines. I hope I am using the right mailing list.
>
> The biggest limitation is the costly emulation of instruction tlbi
> vmalls12e1is in KVM. The cost is exponentially proportional to the IPA size
> exposed by KVM for VM hosting Xen. If I reduce the IPA size to 40-bits in
> KVM, then this issue is not much observable but with the IPA size of 48-bits,
> it is 256x more costly than the former one. Xen uses this instruction too
> frequently and this instruction is trapped and emulated by KVM, and
> performance is not as good as on bare-metal hardware. With 48-bit IPA, it can
> take up to 200 minutes for domu creation with just 128M RAM. I have
> identified two places in Xen which are problematic w.r.t the usage of this
> instruction and hoping to reduce the frequency of this instruction or use a
> more relevant TLBI instruction instead of invalidating whole stage-1 and
> stage-2 translations.
Why the exponential scaling? It should be possible for KVM to fall
back to a full TLB flush, which should be O(1) in the size of the
address space. It might have terrible constant factors though.
--
Sincerely,
Demi Marie Obenour (she/her/hers) Attachment:
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