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Wed, 05 Jul 2023 02:23:10 -0700 (PDT) MIME-Version: 1.0 References: <20230704075054.3344915-1-stevensd@google.com> <20230704075054.3344915-3-stevensd@google.com> <20230705031002.xrxk42hli6oavtlt@linux.intel.com> In-Reply-To: <20230705031002.xrxk42hli6oavtlt@linux.intel.com> From: David Stevens Date: Wed, 5 Jul 2023 18:22:59 +0900 Message-ID: Subject: Re: [PATCH v7 2/8] KVM: Introduce __kvm_follow_pfn function To: Yu Zhang Cc: Sean Christopherson , Marc Zyngier , Michael Ellerman , Peter Xu , linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, kvm@vger.kernel.org Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Spam-Status: No, score=-2.1 required=5.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,RCVD_IN_DNSWL_NONE, SPF_HELO_NONE,SPF_PASS,T_SCC_BODY_TEXT_LINE,URIBL_BLOCKED autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on lindbergh.monkeyblade.net Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jul 5, 2023 at 12:10=E2=80=AFPM Yu Zhang wrote: > > > @@ -2514,35 +2512,26 @@ static bool hva_to_pfn_fast(unsigned long addr,= bool write_fault, > > * The slow path to get the pfn of the specified host virtual address, > > * 1 indicates success, -errno is returned if error is detected. > > */ > > -static int hva_to_pfn_slow(unsigned long addr, bool *async, bool write= _fault, > > - bool interruptible, bool *writable, kvm_pfn_t = *pfn) > > +static int hva_to_pfn_slow(struct kvm_follow_pfn *foll, kvm_pfn_t *pfn= ) > > { > > - unsigned int flags =3D FOLL_HWPOISON; > > + unsigned int flags =3D FOLL_HWPOISON | FOLL_GET | foll->flags; > > struct page *page; > > int npages; > > > > might_sleep(); > > > > - if (writable) > > - *writable =3D write_fault; > > - > > - if (write_fault) > > - flags |=3D FOLL_WRITE; > > - if (async) > > - flags |=3D FOLL_NOWAIT; > > - if (interruptible) > > - flags |=3D FOLL_INTERRUPTIBLE; > > - > > - npages =3D get_user_pages_unlocked(addr, 1, &page, flags); > > + npages =3D get_user_pages_unlocked(foll->hva, 1, &page, flags); > > if (npages !=3D 1) > > return npages; > > > > + foll->writable =3D (foll->flags & FOLL_WRITE) && foll->allow_writ= e_mapping; > > + > > /* map read fault as writable if possible */ > > - if (unlikely(!write_fault) && writable) { > > + if (unlikely(!foll->writable) && foll->allow_write_mapping) { > > I guess !foll->writable should be !(foll->flags & FOLL_WRITE) here. The two statements are logically equivalent, although I guess using !(foll->flags & FOLL_WRITE) may be a little clearer, if a little more verbose. > > struct page *wpage; > > > > - if (get_user_page_fast_only(addr, FOLL_WRITE, &wpage)) { > > - *writable =3D true; > > + if (get_user_page_fast_only(foll->hva, FOLL_WRITE, &wpage= )) { > > + foll->writable =3D true; > > put_page(page); > > page =3D wpage; > > } > > @@ -2572,23 +2561,23 @@ static int kvm_try_get_pfn(kvm_pfn_t pfn) > > return get_page_unless_zero(page); > > } > > > ... > > > +kvm_pfn_t __gfn_to_pfn_memslot(const struct kvm_memory_slot *slot, gfn= _t gfn, > > + bool atomic, bool interruptible, bool *asy= nc, > > + bool write_fault, bool *writable, hva_t *h= va) > > +{ > > + kvm_pfn_t pfn; > > + struct kvm_follow_pfn foll =3D { > > + .slot =3D slot, > > + .gfn =3D gfn, > > + .flags =3D 0, > > + .atomic =3D atomic, > > + .allow_write_mapping =3D !!writable, > > + }; > > + > > + if (write_fault) > > + foll.flags |=3D FOLL_WRITE; > > + if (async) > > + foll.flags |=3D FOLL_NOWAIT; > > + if (interruptible) > > + foll.flags |=3D FOLL_INTERRUPTIBLE; > > + > > + pfn =3D __kvm_follow_pfn(&foll); > > + if (pfn =3D=3D KVM_PFN_ERR_NEEDS_IO) { > > Could we just use KVM_PFN_ERR_FAULT and foll.flags here? I.e., > if (pfn =3D=3D KVM_PFN_ERR_FAULT && (foll.flags & FOLL_NOWAIT))? > Setting pfn to KVM_PFN_ERR_NEEDS_IO just to indicate an async fault > seems unnecessary. There are the cases where the fault does not fall within a vma or when the target vma's flags don't support the fault's access permissions. In those cases, continuing to try to resolve the fault won't cause problems per-se, but it's wasteful and a bit confusing. Having hva_to_pfn detect whether or not it may be possible to resolve the fault asynchronously and return KVM_PFN_ERR_NEEDS_IO if so seems like a good idea. It also matches what the existing code does. -David