From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andrea Arcangeli Subject: Re: [PATCH 2/2] TESTING! KVM: x86: add invalidate_range mmu notifier Date: Sun, 3 Dec 2017 18:24:47 +0100 Message-ID: <20171203172447.GQ8063@redhat.com> References: <20171130161933.GB1606@flask> <20171130180546.4331-1-rkrcmar@redhat.com> <20171130180546.4331-2-rkrcmar@redhat.com> <4e0b6e81-b987-487e-b582-4d61aec9252d@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Cc: Radim =?utf-8?B?S3LEjW3DocWZ?= , Fabian =?iso-8859-1?Q?Gr=FCnbichler?= , linux-kernel@vger.kernel.org, linux-mm@kvack.org, kvm@vger.kernel.org, =?iso-8859-1?B?Suly9G1l?= Glisse To: Paolo Bonzini Return-path: Content-Disposition: inline In-Reply-To: <4e0b6e81-b987-487e-b582-4d61aec9252d@redhat.com> Sender: owner-linux-mm@kvack.org List-Id: kvm.vger.kernel.org On Fri, Dec 01, 2017 at 04:15:37PM +0100, Paolo Bonzini wrote: > On 30/11/2017 19:05, Radim Krčmář wrote: > > Does roughly what kvm_mmu_notifier_invalidate_page did before. > > > > I am not certain why this would be needed. It might mean that we have > > another bug with start/end or just that I missed something. > > I don't think this is needed, because we don't have shared page tables. > My understanding is that without shared page tables, you can assume that > all page modifications go through invalidate_range_start/end. With > shared page tables, there are additional TLB flushes to take care of, > which require invalidate_range. Agreed, invalidate_range only is ever needed if you the secondary MMU (i.e. KVM) shares the same pagetables of the primary MMU in the host. Only in such case we need a special secondary MMU invalidate in the tlb gather before the page is freed because there's no way to block the secondary MMU from walking the host pagetables in invalidate_range_start. In KVM case the secondary MMU always go through the shadow pagetables, so all shadow pagetable invalidates can happen in invalidate_range_start and patch 2/2 is not needed here. Note that the host kernel could have always decided to call invalidate_range_start/end and never to call invalidate_page even before invalidate_page was removed. So the problem in practice could only be noticed after the removal of invalidate_page of course, but in more theoretical terms 1/2 is actually fixing a longstanding bug. The removal of invalidate_page made the lack of kvm_arch_mmu_notifier_invalidate_page call in invalidate_range_start more apparent. Thanks, Andrea -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org