linux-kernel - Re: [PATCH 4/4] KVM: x86/mmu: Factor out tdp_iter_return_to

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Date:   Fri, 12 Mar 2021 08:35:24 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Ben Gardon <bgardon@...gle.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Peter Shier <pshier@...gle.com>,
        Jim Mattson <jmattson@...gle.com>
Subject: Re: [PATCH 4/4] KVM: x86/mmu: Factor out tdp_iter_return_to_root

On Thu, Mar 11, 2021, Ben Gardon wrote:
> In tdp_mmu_iter_cond_resched there is a call to tdp_iter_start which
> causes the iterator to continue its walk over the paging structure from
> the root. This is needed after a yield as paging structure could have
> been freed in the interim.
> 
> The tdp_iter_start call is not very clear and something of a hack. It
> requires exposing tdp_iter fields not used elsewhere in tdp_mmu.c and
> the effect is not obvious from the function name. Factor a more aptly
> named function out of tdp_iter_start and call it from
> tdp_mmu_iter_cond_resched and tdp_iter_start.

What about calling it tdp_iter_restart()?  Or tdp_iter_resume()?  Or something
like tdp_iter_restart_at_next() if we want it to give a hint that the next_last
thing is where it restarts.

I think I like tdp_iter_restart() the best.  It'd be easy enough to add a
function comment clarifying from where it restarts, and IMO the resulting code
in tdp_mmu_iter_cond_resched() is the most intutive, e.g. it makes it very clear
that the walk is being restarted in some capacity after yielding.