linux-kernel - Re: [PATCH v13 20/35] KVM: x86/mmu: Handle page fault for private memory

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Date:   Mon, 6 Nov 2023 21:29:10 +0800
From:   Xu Yilun <yilun.xu@...ux.intel.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <seanjc@...gle.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>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>, 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-kernel@...r.kernel.org, Xiaoyao Li <xiaoyao.li@...el.com>,
        Xu Yilun <yilun.xu@...el.com>,
        Chao Peng <chao.p.peng@...ux.intel.com>,
        Fuad Tabba <tabba@...gle.com>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        Anish Moorthy <amoorthy@...gle.com>,
        David Matlack <dmatlack@...gle.com>,
        Yu Zhang <yu.c.zhang@...ux.intel.com>,
        Isaku Yamahata <isaku.yamahata@...el.com>,
        Mickaël Salaün <mic@...ikod.net>,
        Vlastimil Babka <vbabka@...e.cz>,
        Vishal Annapurve <vannapurve@...gle.com>,
        Ackerley Tng <ackerleytng@...gle.com>,
        Maciej Szmigiero <mail@...iej.szmigiero.name>,
        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: [PATCH v13 20/35] KVM: x86/mmu: Handle page fault for private
 memory

On Sun, Nov 05, 2023 at 05:19:36PM +0100, Paolo Bonzini wrote:
> On Sun, Nov 5, 2023 at 2:04 PM Xu Yilun <yilun.xu@...ux.intel.com> wrote:
> >
> > > +static void kvm_mmu_prepare_memory_fault_exit(struct kvm_vcpu *vcpu,
> > > +                                           struct kvm_page_fault *fault)
> > > +{
> > > +     kvm_prepare_memory_fault_exit(vcpu, fault->gfn << PAGE_SHIFT,
> > > +                                   PAGE_SIZE, fault->write, fault->exec,
> > > +                                   fault->is_private);
> > > +}
> > > +
> > > +static int kvm_faultin_pfn_private(struct kvm_vcpu *vcpu,
> > > +                                struct kvm_page_fault *fault)
> > > +{
> > > +     int max_order, r;
> > > +
> > > +     if (!kvm_slot_can_be_private(fault->slot)) {
> > > +             kvm_mmu_prepare_memory_fault_exit(vcpu, fault);
> > > +             return -EFAULT;
> > > +     }
> > > +
> > > +     r = kvm_gmem_get_pfn(vcpu->kvm, fault->slot, fault->gfn, &fault->pfn,
> > > +                          &max_order);
> > > +     if (r) {
> > > +             kvm_mmu_prepare_memory_fault_exit(vcpu, fault);
> > > +             return r;
> >
> > Why report KVM_EXIT_MEMORY_FAULT here? even with a ret != -EFAULT?
> 
> The cases are EFAULT, EHWPOISON (which can report
> KVM_EXIT_MEMORY_FAULT) and ENOMEM. I think it's fine
> that even -ENOMEM can return KVM_EXIT_MEMORY_FAULT,
> and it doesn't violate the documentation.  The docs tell you "what
> can you do if error if EFAULT or EHWPOISON?"; they don't
> exclude that other errnos result in KVM_EXIT_MEMORY_FAULT,
> it's just that you're not supposed to look at it

Thanks, it's OK for ENOMEM + KVM_EXIT_MEMORY_FAULT.

Another concern is, now 3 places to report EFAULT + KVM_EXIT_MEMORY_FAULT:

  if (!kvm_slot_can_be_private(fault->slot)) {
	kvm_mmu_prepare_memory_fault_exit(vcpu, fault);
	return -EFAULT;
  }

  file = kvm_gmem_get_file(slot);
  if (!file)
	return -EFAULT;

  if (fault->is_private != kvm_mem_is_private(vcpu->kvm, fault->gfn)) {
	kvm_mmu_prepare_memory_fault_exit(vcpu, fault);
	return -EFAULT;
  }

They are different cases, and seems userspace should handle them
differently, but not enough information to distinguish them.

Thanks,
Yilun

> 
> Paolo
> 
>