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

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Message-ID: <20240319163309.GG1645738@ls.amr.corp.intel.com>
Date: Tue, 19 Mar 2024 09:33:09 -0700
From: Isaku Yamahata <isaku.yamahata@...el.com>
To: isaku.yamahata@...el.com
Cc: David Matlack <dmatlack@...gle.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>
Subject: Re: [RFC PATCH 0/8] KVM: Prepopulate guest memory API

On Wed, Mar 06, 2024 at 06:09:54PM -0800,
Isaku Yamahata <isaku.yamahata@...ux.intel.com> wrote:

> 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.

Any input?  If no further input, I assume the primary use case is pre-population
before guest running.
-- 
Isaku Yamahata <isaku.yamahata@...el.com>