linux-kernel - Re: [RFC PATCH v11 01/29] KVM: Wrap kvm_gfn

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Date:   Wed, 19 Jul 2023 08:39:09 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Jarkko Sakkinen <jarkko@...nel.org>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, Marc Zyngier <maz@...nel.org>,
        Oliver Upton <oliver.upton@...ux.dev>,
        Huacai Chen <chenhuacai@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Anup Patel <anup@...infault.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Paul Moore <paul@...l-moore.com>,
        James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>, kvm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
        linux-mips@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        kvm-riscv@...ts.infradead.org, linux-riscv@...ts.infradead.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-security-module@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Chao Peng <chao.p.peng@...ux.intel.com>,
        Fuad Tabba <tabba@...gle.com>,
        Yu Zhang <yu.c.zhang@...ux.intel.com>,
        Vishal Annapurve <vannapurve@...gle.com>,
        Ackerley Tng <ackerleytng@...gle.com>,
        Maciej Szmigiero <mail@...iej.szmigiero.name>,
        Vlastimil Babka <vbabka@...e.cz>,
        David Hildenbrand <david@...hat.com>,
        Quentin Perret <qperret@...gle.com>,
        Michael Roth <michael.roth@....com>,
        Wang <wei.w.wang@...el.com>,
        Liam Merwick <liam.merwick@...cle.com>,
        Isaku Yamahata <isaku.yamahata@...il.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [RFC PATCH v11 01/29] KVM: Wrap kvm_gfn_range.pte in a per-action union

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.