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Wed, 13 Sep 2023 10:46:12 -0700 (PDT) Date: Wed, 13 Sep 2023 10:46:11 -0700 In-Reply-To: <852b6fa117bf3767a99353d908bc566a5dd9c61a.1694599703.git.isaku.yamahata@intel.com> Mime-Version: 1.0 References: <852b6fa117bf3767a99353d908bc566a5dd9c61a.1694599703.git.isaku.yamahata@intel.com> Message-ID: Subject: Re: [RFC PATCH 4/6] KVM: guest_memfd: Implemnet bmap inode operation From: Sean Christopherson To: isaku.yamahata@intel.com Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org, isaku.yamahata@gmail.com, Michael Roth , Paolo Bonzini , erdemaktas@google.com, Sagi Shahar , David Matlack , Kai Huang , Zhi Wang , chen.bo@intel.com, linux-coco@lists.linux.dev, Chao Peng , Ackerley Tng , Vishal Annapurve , Yuan Yao , Jarkko Sakkinen , Xu Yilun , Quentin Perret , wei.w.wang@intel.com, Fuad Tabba Content-Type: text/plain; charset="us-ascii" Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On Wed, Sep 13, 2023, isaku.yamahata@intel.com wrote: > From: Isaku Yamahata > > To inject memory failure, physical address of the page is needed. > Implement bmap() method to convert the file offset into physical address. > > Signed-off-by: Isaku Yamahata > --- > virt/kvm/Kconfig | 4 ++++ > virt/kvm/guest_mem.c | 28 ++++++++++++++++++++++++++++ > 2 files changed, 32 insertions(+) > > diff --git a/virt/kvm/Kconfig b/virt/kvm/Kconfig > index 624df45baff0..eb008f0e7cc3 100644 > --- a/virt/kvm/Kconfig > +++ b/virt/kvm/Kconfig > @@ -115,3 +115,7 @@ config KVM_GENERIC_PRIVATE_MEM > > config HAVE_GENERIC_PRIVATE_MEM_HANDLE_ERROR > bool > + > +config KVM_GENERIC_PRIVATE_MEM_BMAP > + depends on KVM_GENERIC_PRIVATE_MEM > + bool > diff --git a/virt/kvm/guest_mem.c b/virt/kvm/guest_mem.c > index 3678287d7c9d..90dfdfab1f8c 100644 > --- a/virt/kvm/guest_mem.c > +++ b/virt/kvm/guest_mem.c > @@ -355,12 +355,40 @@ static int kvm_gmem_error_page(struct address_space *mapping, struct page *page) > return MF_DELAYED; > } > > +#ifdef CONFIG_KVM_GENERIC_PRIVATE_MEM_BMAP > +static sector_t kvm_gmem_bmap(struct address_space *mapping, sector_t block) > +{ > + struct folio *folio; > + sector_t pfn = 0; > + > + filemap_invalidate_lock_shared(mapping); > + > + if (block << PAGE_SHIFT > i_size_read(mapping->host)) > + goto out; > + > + folio = filemap_get_folio(mapping, block); > + if (IS_ERR_OR_NULL(folio)) > + goto out; > + > + pfn = folio_pfn(folio) + (block - folio->index); > + folio_put(folio); > + > +out: > + filemap_invalidate_unlock_shared(mapping); > + return pfn; IIUC, hijacking bmap() is a gigantic hack to propagate a host pfn to userspace without adding a new ioctl() or syscall. If we want to support target injection, I would much, much rather add a KVM ioctl(), e.g. to let userspace inject errors for a gfn. Returning a pfn for something that AFAICT has nothing to do with pfns is gross, e.g. the whole "0 is the error code" thing is technically wrong because '0' is a perfectly valid pfn. My vote is to drop this and not extend the injection information for the initial merge, i.e. rely on point testing to verify kvm_gmem_error_page(), and defer adding uAPI to let selftests inject errors. > + > +} > +#endif > + > static const struct address_space_operations kvm_gmem_aops = { > .dirty_folio = noop_dirty_folio, > #ifdef CONFIG_MIGRATION > .migrate_folio = kvm_gmem_migrate_folio, > #endif > .error_remove_page = kvm_gmem_error_page, > +#ifdef CONFIG_KVM_GENERIC_PRIVATE_MEM_BMAP > + .bmap = kvm_gmem_bmap, > +#endif > }; > > static int kvm_gmem_getattr(struct mnt_idmap *idmap, > -- > 2.25.1 >