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

[Paolo Bonzini]

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@xxxxxxxxxx> 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@xxxxxxxxxx/

See in particular

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

Paolo