linux-kernel - Re: [RFC PATCH 08/21] KVM: TDX: Increase/decrease folio ref for huge pages

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Message-ID: <69c4b586bf6f6b1e13ad59113730548c84424ecf.camel@intel.com>
Date: Tue, 1 Jul 2025 15:42:28 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "Annapurve, Vishal" <vannapurve@...gle.com>, "Zhao, Yan Y"
	<yan.y.zhao@...el.com>
CC: "Shutemov, Kirill" <kirill.shutemov@...el.com>, "Li, Xiaoyao"
	<xiaoyao.li@...el.com>, "Du, Fan" <fan.du@...el.com>, "Hansen, Dave"
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	<tabba@...gle.com>, "vbabka@...e.cz" <vbabka@...e.cz>, "kvm@...r.kernel.org"
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Subject: Re: [RFC PATCH 08/21] KVM: TDX: Increase/decrease folio ref for huge
 pages

On Tue, 2025-07-01 at 07:02 -0700, Vishal Annapurve wrote:
> > guest_memfd will have to ensure that pages are unmapped from secure
> > IOMMU pagetables before allowing them to be used by the host.
> > 
> > If secure IOMMU pagetables unmapping fails, I would assume it fails in
> > the similar category of rare "KVM/TDX module/IOMMUFD" bug and I think
> > it makes sense to do the same tdx_buggy_shutdown() with such failures
> > as well.
> 
> In addition we will need a way to fail all further Secure IOMMU table
> walks or some way to stop the active secure DMA by unbinding all the
> TDIs. Maybe such scenarios warrant a BUG_ON() if recovery is not
> possible as possibly any or all of the KVM/IOMMUFD/TDX module can't be
> trusted for reliable functionality anymore.

I mentioned this on another thread. Normal kernel BUG_ON()'s need extreme
justification. As long as the system might survive, they shouldn't be used.