From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753580AbbDHMQz (ORCPT ); Wed, 8 Apr 2015 08:16:55 -0400 Received: from mx1.redhat.com ([209.132.183.28]:39864 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751300AbbDHMQx (ORCPT ); Wed, 8 Apr 2015 08:16:53 -0400 Date: Wed, 8 Apr 2015 14:16:48 +0200 From: Radim =?utf-8?B?S3LEjW3DocWZ?= To: Paolo Bonzini Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org Subject: [PATCH] KVM: use slowpath for cross page cached accesses Message-ID: <20150408121648.GA3519@potion.brq.redhat.com> References: <1428438897-22206-1-git-send-email-rkrcmar@redhat.com> <5524EBB7.3080906@redhat.com> <20150408092611.GA2164@potion.brq.redhat.com> <55250643.3090402@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <55250643.3090402@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 2015-04-08 12:43+0200, Paolo Bonzini: > On 08/04/2015 11:26, Radim Krčmář wrote: >> 2015-04-08 10:49+0200, Paolo Bonzini: >>> On 07/04/2015 22:34, Radim Krčmář wrote: >>>> We dirtied only one page because writes originally couldn't span more. >>>> Use improved syntax for '>> PAGE_SHIFT' while at it. >>>> >>>> Fixes: 8f964525a121 ("KVM: Allow cross page reads and writes from cached translations.") >>>> Signed-off-by: Radim Krčmář >>> >>> Cross-page reads and writes should never get here; they have >>> ghc->memslot set to NULL and go through the slow path in kvm_write_guest. >> >> Only cross-memslot writes have NULL memslot. > > The power of wrong comments... > > Considering how kvm_gfn_to_hva_cache_init is used (one 1-byte field, two > 4-byte fields, one 28-bytes struct that is 32-bytes aligned, one > 32-bytes field that is in practice cacheline-aligned), I wonder if we > should just use ghc->memslot = NULL for cross page writes. This would > bypass the bug you are fixing here, and avoid worries about partial writes. Good idea, and it could make those comments right :) (Though in general, I prefer less constraints on APIs ...) Partial writes would be a pain; copy_to_user API does not define which bytes were not written. I think the write can't fail mid-page, which makes our implementation ok, but I still worry a bit about it. Anyway, here's the patch: ---8<--- kvm_write_guest_cached() does not mark all written pages as dirty and code comments in kvm_gfn_to_hva_cache_init() talk about NULL memslot with cross page accesses. Fix all the easy way. The check is '<= 1' to have the same result for 'len = 0' cache anywhere in the page. (nr_pages_needed is 0 on page boundary.) Fixes: 8f964525a121 ("KVM: Allow cross page reads and writes from cached translations.") Signed-off-by: Radim Krčmář --- virt/kvm/kvm_main.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c index aadef264bed1..f3dc641f9640 100644 --- a/virt/kvm/kvm_main.c +++ b/virt/kvm/kvm_main.c @@ -1637,8 +1637,8 @@ int kvm_gfn_to_hva_cache_init(struct kvm *kvm, struct gfn_to_hva_cache *ghc, ghc->generation = slots->generation; ghc->len = len; ghc->memslot = gfn_to_memslot(kvm, start_gfn); - ghc->hva = gfn_to_hva_many(ghc->memslot, start_gfn, &nr_pages_avail); - if (!kvm_is_error_hva(ghc->hva) && nr_pages_avail >= nr_pages_needed) { + ghc->hva = gfn_to_hva_many(ghc->memslot, start_gfn, NULL); + if (!kvm_is_error_hva(ghc->hva) && nr_pages_needed <= 1) { ghc->hva += offset; } else { /*