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

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Message-ID: <20240307020954.GG368614@ls.amr.corp.intel.com>
Date: Wed, 6 Mar 2024 18:09:54 -0800
From: Isaku Yamahata <isaku.yamahata@...ux.intel.com>
To: David Matlack <dmatlack@...gle.com>
Cc: isaku.yamahata@...el.com, 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>,
	isaku.yamahata@...ux.intel.com
Subject: Re: [RFC PATCH 0/8] KVM: Prepopulate guest memory API

On Wed, Mar 06, 2024 at 04:53:41PM -0800,
David Matlack <dmatlack@...gle.com> wrote:

> 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?

As you pointed out in other mail, legacy KVM MMU support or guest-mode will be
troublesome.  The use case I supposed is pre-population before guest runs, the
guest-mode wouldn't matter. I didn't add explicit check for it, though.

Any use case while vcpus running?


> 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.

If the use case for default VM or sw-proteced VM is to avoid excessive kvm page
fault at guest boot, error on guest-mode or disabled TDP wouldn't matter.
-- 
Isaku Yamahata <isaku.yamahata@...ux.intel.com>