linux-kernel - Re: [RFC PATCH 09/12] KVM: TDX: Fold tdx_mem_page_record_premap

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Message-ID: <aLC7k65GpIL-2Hk5@google.com>
Date: Thu, 28 Aug 2025 13:26:59 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Rick P Edgecombe <rick.p.edgecombe@...el.com>
Cc: Yan Y Zhao <yan.y.zhao@...el.com>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>, 
	"pbonzini@...hat.com" <pbonzini@...hat.com>, Vishal Annapurve <vannapurve@...gle.com>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, 
	"michael.roth@....com" <michael.roth@....com>, Ira Weiny <ira.weiny@...el.com>
Subject: Re: [RFC PATCH 09/12] KVM: TDX: Fold tdx_mem_page_record_premap_cnt()
 into its sole caller

On Thu, Aug 28, 2025, Rick P Edgecombe wrote:
> On Thu, 2025-08-28 at 10:00 -0700, Sean Christopherson wrote:
> > > tdx_td_finalize() now just returns -EINVAL in case of nr_premapped being !0.
> > > KVM_BUG_ON/WARN_ON should be also ok.
> > 
> > Ok, so I vaguely recall that I may have pushed back on using a scratch field in
> > "struct kvm_tdx" for temporary data (or maybe it was abusing vcpus[0] that I
> > disliked?), but what we ended up with is far worse.
> 
> I think it was also that the tdh_mr_extend() loop was too heavyweight for the
> fault path. But that was before we got to the kick+lock stuff.

Me confused.  This is pre-boot, not the normal fault path, i.e. blocking other
operations is not a concern.

If tdh_mr_extend() is too heavy for a non-preemptible section, then the current
code is also broken in the sense that there are no cond_resched() calls.  The
vast majority of TDX hosts will be using non-preemptible kernels, so without an
explicit cond_resched(), there's no practical difference between extending the
measurement under mmu_lock versus outside of mmu_lock.

_If_ we need/want to do tdh_mr_extend() outside of mmu_lock, we can and should
still do tdh_mem_page_add() under mmu_lock.