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Re: About two-dimensional page translation (e.g., Intel EPT) and shadow

From: harry harry
Subject: Re: About two-dimensional page translation (e.g., Intel EPT) and shadow page table in Linux QEMU/KVM
Date: Wed, 14 Jul 2021 00:30:12 -0500

Dear Sean,

Thanks for the comments!

> Heh, because the MMUs are all per-vCPU, it actually wouldn't be that much 
> effort
> beyond supporting !TDP and TDP for different VMs...

Sorry, may I know what do you mean by "MMUs are all per-vCPU"? Do you
mean the MMUs walk the page tables of each vCPU?

> ...but supporting !TDP and TDP in a single KVM instance isn't going to happen.
> It's certainly possible, but comes with a very high complexity cost, and 
> likely
> even performance costs.

For one KVM instance, I think it might be possible to let several
physical cores use !TDP and other cores use TDP but I am not sure
about the implementation complexity.

> The more sane way to support !TDP and TDP on a single host would be to support
> multiple instances of KVM, e.g. /dev/kvm0, /dev/kvm1, etc...  Being able to 
> use
> !TDP and TDP isn't strong justification for the work required, but supporting
> multiple KVM instances would allow upgrading KVM without having to migrate VMs
> off the host, which is very desirable.  If multiple KVM instances are 
> supported,
> running !TDP and TDP KVM instances should Just Work.

Yes, for different KVM instances, it may be much easier but there
might be some other issues, e.g., communication overhead between
different instances. I think the upgrading idea is great but is very
limited to local upgrading.


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