linux-kernel - Re: [RFC PATCH 0/8] KVM: Prepopulate guest memory API

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Message-ID: <ZekQFdPlU7RDVt-B@google.com>
Date: Wed, 6 Mar 2024 16:53:41 -0800
From: David Matlack <dmatlack@...gle.com>
To: isaku.yamahata@...el.com
Cc: kvm@...r.kernel.org, isaku.yamahata@...il.com,
	linux-kernel@...r.kernel.org,
	Sean Christopherson <seanjc@...gle.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Michael Roth <michael.roth@....com>,
	Federico Parola <federico.parola@...ito.it>
Subject: Re: [RFC PATCH 0/8] KVM: Prepopulate guest memory API

On 2024-03-01 09:28 AM, isaku.yamahata@...el.com wrote:
> From: Isaku Yamahata <isaku.yamahata@...el.com>
> 
> Implementation:
> - x86 KVM MMU
>   In x86 KVM MMU, I chose to use kvm_mmu_do_page_fault().  It's not confined to
>   KVM TDP MMU.  We can restrict it to KVM TDP MMU and introduce an optimized
>   version.

Restricting to TDP MMU seems like a good idea. But I'm not quite sure
how to reliably do that from a vCPU context. Checking for TDP being
enabled is easy, but what if the vCPU is in guest-mode?

Perhaps we can just return an error out to userspace if the vCPU is in
guest-mode or TDP is disabled, and make it userspace's problem to do
memory mapping before loading any vCPU state.