Re: [RFC PATCH v11 01/29] KVM: Wrap kvm_gfn_range.pte in a per-action union

From: Sean Christopherson <seanjc@google•com>
To: Jarkko Sakkinen <jarkko@kernel•org>
Cc: Paolo Bonzini <pbonzini@redhat•com>,
	Marc Zyngier <maz@kernel•org>,
	 Oliver Upton <oliver.upton@linux•dev>,
	Huacai Chen <chenhuacai@kernel•org>,
	 Michael Ellerman <mpe@ellerman•id.au>,
	Anup Patel <anup@brainfault•org>,
	 Paul Walmsley <paul.walmsley@sifive•com>,
	Palmer Dabbelt <palmer@dabbelt•com>,
	 Albert Ou <aou@eecs•berkeley.edu>,
	"Matthew Wilcox (Oracle)" <willy@infradead•org>,
	 Andrew Morton <akpm@linux-foundation•org>,
	Paul Moore <paul@paul-moore•com>,
	 James Morris <jmorris@namei•org>,
	"Serge E. Hallyn" <serge@hallyn•com>,
	kvm@vger•kernel.org,  linux-arm-kernel@lists•infradead.org,
	kvmarm@lists•linux.dev,  linux-mips@vger•kernel.org,
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	linux-kernel@vger•kernel.org,
	 Chao Peng <chao.p.peng@linux•intel.com>,
	Fuad Tabba <tabba@google•com>,
	 Yu Zhang <yu.c.zhang@linux•intel.com>,
	Vishal Annapurve <vannapurve@google•com>,
	 Ackerley Tng <ackerleytng@google•com>,
	Maciej Szmigiero <mail@maciej•szmigiero.name>,
	 Vlastimil Babka <vbabka@suse•cz>,
	David Hildenbrand <david@redhat•com>,
	Quentin Perret <qperret@google•com>,
	 Michael Roth <michael.roth@amd•com>, Wang <wei.w.wang@intel•com>,
	 Liam Merwick <liam.merwick@oracle•com>,
	Isaku Yamahata <isaku.yamahata@gmail•com>,
	 "Kirill A . Shutemov" <kirill.shutemov@linux•intel.com>
Subject: Re: [RFC PATCH v11 01/29] KVM: Wrap kvm_gfn_range.pte in a per-action union
Date: Wed, 19 Jul 2023 08:39:09 -0700	[thread overview]
Message-ID: <ZLgDnYJUX/QR9UJi@google.com> (raw)
In-Reply-To: <CU66VMG4IKSD.33KF2CEZJ2I1@suppilovahvero>

On Wed, Jul 19, 2023, Jarkko Sakkinen wrote:
> On Wed Jul 19, 2023 at 2:44 AM EEST, Sean Christopherson wrote:
> >  	/* Huge pages aren't expected to be modified without first being zapped. */
> > -	WARN_ON(pte_huge(range->pte) || range->start + 1 != range->end);
> > +	WARN_ON(pte_huge(range->arg.pte) || range->start + 1 != range->end);
> 
> Not familiar with this code. Just checking whether whether instead
> pr_{warn,err}()

The "full" WARN is desirable, this is effecitvely an assert on the contract between
the primary MMU, generic KVM code, and x86's TDP MMU.  The .change_pte() mmu_notifier
callback doesn't allow for hugepages, i.e. it's a (likely fatal) kernel bug if a
hugepage is encountered at this point.  Ditto for the "start + 1 == end" check,
if that fails then generic KVM likely has a fatal bug.

> combined with return false would be a more graceful option?

The return value communicates whether or not a TLB flush is needed, not whether
or not the operation was successful, i.e. there is no way to cancel the unexpected
PTE change.

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