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Date:   Wed, 19 Jul 2023 08:39:09 -0700
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Subject: 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, linuxppc-dev@lists.ozlabs.org,
        kvm-riscv@lists.infradead.org, linux-riscv@lists.infradead.org,
        linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
        linux-security-module@vger.kernel.org,
        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>
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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.