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

From: Sean Christopherson
Subject: Re: About two-dimensional page translation (e.g., Intel EPT) and shadow page table in Linux QEMU/KVM
Date: Wed, 14 Jul 2021 17:47:42 +0000

On Wed, Jul 14, 2021, harry harry wrote:
> > 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?

No, each vCPU has its own MMU instance, where an "MMU instance" is (mostly) a 
construct.  Per-vCPU MMU instances are necessary because each vCPU has its own
relevant state, e.g. CR0, CR4, EFER, etc..., that affects the MMU instance in
some way.  E.g. the MMU instance is used to walk guest page tables when
translating GVA->GPA for emulation, so per-vCPU MMUs are necessary even when
using TDP.

However, shadow/TDP PTEs are shared between compatible MMU instances.  E.g. in
the common case where all vCPUs in a VM use identical settings, there will
effectively be a single set of TDP page tables shared by all vCPUs.

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