linux-kernel - Re: [RFC PATCH 08/12] KVM: TDX: Use atomic64_dec

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Message-ID: <aLDZHf55rz1W0n6b@google.com>
Date: Thu, 28 Aug 2025 15:33:01 -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 08/12] KVM: TDX: Use atomic64_dec_return() instead of
 a poor equivalent

On Thu, Aug 28, 2025, Rick P Edgecombe wrote:
> On Thu, 2025-08-28 at 14:48 +0800, Yan Zhao wrote:
> > Hmm, I still think it's safer to keep the nr_premapped to detect any unexpected
> > code change.
> 
> When I checking patch 6 I saw how many more KVM_BUG_ON()s we ended up with in
> TDX code compared to the rest of KVM. (even after we dropped a bunch during
> development) We have to differentiate from good safety, and "safety" that is
> really just propping up brittle code. Each KVM_BUG_ON() is a hint that there
> might be design issues.

Nah, I think we're good.  The majority of the asserts are on SEAMCALLs, and those
are no different than the WARN_ONCE() in vmx_insn_failed(), just spread out to
individual call sites.

Once those are out of the numbers are entirely reasonable (WARNs and KVM_BUG_ON
are both assertions against bugs, one is just guaranteed to be fatal to the VM).

  $ git grep -e KVM_BUG_ON -e WARN_ vmx/tdx.c | wc -l
  25
  $ git grep -e KVM_BUG_ON -e WARN_  | wc -l
  459