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

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Message-ID: <Yid+ky+1sPJpyelQ@google.com>
Date:   Tue, 8 Mar 2022 16:04:35 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Hao Peng <flyingpenghao@...il.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kvm: x86: Improve virtual machine startup performance

On Tue, Mar 08, 2022, Paolo Bonzini wrote:
> On 3/8/22 03:03, Sean Christopherson wrote:
> > 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 :-)
> 
> This is exactly what I have posted a few weeks ago:
> 
> https://patchew.org/linux/20220221162243.683208-1-pbonzini@redhat.com/

Heh, guess who's woefully behind on reviews...  I'll respond to those threads.
Thanks for the heads up!

> See in particular
> 
>   KVM: nVMX/nSVM: do not monkey-patch inject_page_fault callback
>   KVM: x86/mmu: initialize constant-value fields just once