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

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Message-ID: <da91ddb191db9e4178e5d1e05a0d2a40505f0d2a.camel@intel.com>
Date: Thu, 28 Aug 2025 23:18:25 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "seanjc@...gle.com" <seanjc@...gle.com>
CC: "kvm@...r.kernel.org" <kvm@...r.kernel.org>, "pbonzini@...hat.com"
	<pbonzini@...hat.com>, "Annapurve, Vishal" <vannapurve@...gle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "Zhao, Yan Y"
	<yan.y.zhao@...el.com>, "michael.roth@....com" <michael.roth@....com>,
	"Weiny, Ira" <ira.weiny@...el.com>
Subject: Re: [RFC PATCH 08/12] KVM: TDX: Use atomic64_dec_return() instead of
 a poor equivalent

On Thu, 2025-08-28 at 15:33 -0700, Sean Christopherson wrote:
> 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

Hmm, ok.