linux-kernel - Re: [PATCH] kvm: x86: Improve virtual machine startup performance

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Message-ID: <Yia5hsoq2ZZJM8gx@google.com>
Date:   Tue, 8 Mar 2022 02:03:50 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Hao Peng <flyingpenghao@...il.com>
Cc:     pbonzini@...hat.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kvm: x86: Improve virtual machine startup performance

On Thu, Mar 03, 2022, Hao Peng wrote:
> On Thu, Mar 3, 2022 at 9:29 AM Sean Christopherson <seanjc@...gle.com> wrote:
> >
> > On Wed, Mar 02, 2022, Hao Peng wrote:
> > > Thanks for pointing this out. However, other than shadow_root_level,
> > > other fields of context will not
> > > change during the entire operation, such as
> > > page_fault/sync_page/direct_map and so on under
> > > the condition of tdp_mmu.
> > > Is this patch still viable after careful confirmation of the fields
> > > that won't be modified?
> >
> > No, passing around the "init" flag is a hack.
> >
> > But, we can achieve what you want simply by initializing the constant data once
> > per vCPU.  There's a _lot_ of state that is constant for a given MMU now that KVM
> > uses separate MMUs for L1 vs. L2 when TDP is enabled.  I should get patches posted
> > tomorrow, just need to test (famous last words).

Famous last words indeed.  Long story short, the patches were mostly easy, but I
wandered deep into a rabbit hole when trying to make ->inject_page_fault() constant
per MMU.  I'll get something posted this week, though exactly what that something is
remains to be seen :-)