linux-kernel - Re: [PATCH v4 00/11] KVM: x86/mmu: Make tdp

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Message-ID: <Y0hxFF6ai3cX8uA+@google.com>
Date:   Thu, 13 Oct 2022 20:12:04 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     David Matlack <dmatlack@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Isaku Yamahata <isaku.yamahata@...el.com>
Subject: Re: [PATCH v4 00/11] KVM: x86/mmu: Make tdp_mmu a read-only parameter

On Thu, Oct 13, 2022, David Matlack wrote:
> On Wed, Oct 12, 2022 at 11:17 AM Sean Christopherson <seanjc@...gle.com> wrote:
> > I'm not dead set against having a dedicated TDP MMU page fault handler, but
> > IMO it's not really better once the TDP MMU vs. shadow MMU is reduced to a
> > static branch, just different.  The read vs. write mmu_lock is the most
> > visible ugliness, and that can be buried in helpers if we really want to
> > make the page fault handler easier on the eyes, e.g.
 
...

> My preference is still separate handlers. When I am reading this code,
> I only care about one path (TDP MMU or Shadow MMU, usually TDP MMU).
> Having separate handlers makes it easy to read since I don't have to
> care about the implementation details of the other MMU.
> 
> And more importantly (but less certain), the TDP MMU fault handler is
> going to diverge further from the Shadow MMU fault handler in the near
> future. i.e. There will be more and more branches in a common fault
> handler, and the value of having a common fault handler diminishes.
> Specifically, to support moving the TDP MMU to common code, the TDP
> MMU is no longer going to topup the same mem caches as the Shadow MMU
> (TDP MMU is not going to use struct kvm_mmu_page), and the TDP MMU
> will probably have its own fast_page_fault() handler eventually.

What if we hold off on the split for the moment, and then revisit the handler when
a common MMU is closer to reality?  I agree that a separate handler makes sense
once things start diverging, but until that happens, supporting two flows instead
of one seems like it would add (minor) maintenance cost without much benefit.

> If we do go the common handler route, I don't prefer the
> direct_page_fault_mmu_lock/unlock() wrapper since it further obscures
> the differences between the 2 MMUs.

Yeah, I don't like the wrappers either.