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Date:   Wed, 19 Jul 2023 10:47:41 -0700
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Subject: Re: [RFC PATCH v11 12/29] KVM: Add KVM_CREATE_GUEST_MEMFD ioctl() for
 guest-specific backing memory
From:   Sean Christopherson <seanjc@google.com>
To:     Vishal Annapurve <vannapurve@google.com>
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>,
        Jarkko Sakkinen <jarkko@kernel.org>,
        Yu Zhang <yu.c.zhang@linux.intel.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, Vishal Annapurve wrote:
> On Tue, Jul 18, 2023 at 4:49=E2=80=AFPM Sean Christopherson <seanjc@googl=
e.com> wrote:
> > ...
> > +static int kvm_gmem_error_page(struct address_space *mapping, struct p=
age *page)
> > +{
> > +       struct list_head *gmem_list =3D &mapping->private_list;
> > +       struct kvm_memory_slot *slot;
> > +       struct kvm_gmem *gmem;
> > +       unsigned long index;
> > +       pgoff_t start, end;
> > +       gfn_t gfn;
> > +
> > +       filemap_invalidate_lock_shared(mapping);
> > +
> > +       start =3D page->index;
> > +       end =3D start + thp_nr_pages(page);
> > +
> > +       list_for_each_entry(gmem, gmem_list, entry) {
> > +               xa_for_each_range(&gmem->bindings, index, slot, start, =
end - 1) {
> > +                       for (gfn =3D start; gfn < end; gfn++) {
> > +                               if (WARN_ON_ONCE(gfn < slot->base_gfn |=
|
> > +                                               gfn >=3D slot->base_gfn=
 + slot->npages))
> > +                                       continue;
> > +
> > +                               /*
> > +                                * FIXME: Tell userspace that the *priv=
ate*
> > +                                * memory encountered an error.
> > +                                */
> > +                               send_sig_mceerr(BUS_MCEERR_AR,
> > +                                               (void __user *)gfn_to_h=
va_memslot(slot, gfn),
> > +                                               PAGE_SHIFT, current);
>=20
> Does it make sense to replicate what happens with MCE handling on
> tmpfs backed guest memory:
> 1) Unmap gpa from guest
> 2) On the next guest EPT fault, exit to userspace to handle/log the
> mce error for the gpa.

Hmm, yes, that would be much better.  Ah, and kvm_gmem_get_pfn() needs to c=
heck
folio_test_hwpoison() and potentially PageHWPoison().  E.g. if the folio is=
 huge,
KVM needs to restrict the mapping to order-0 (target page isn't poisoned), =
or
return KVM_PFN_ERR_HWPOISON (taget page IS poisoned).

Alternatively, KVM could punch a hole in kvm_gmem_error_page(), but I don't=
 think
we want to do that because that would prevent forwarding the #MC to the gue=
st.