From mboxrd@z Thu Jan 1 00:00:00 1970 From: Xiao Guangrong Subject: Re: [PATCH v3 2/9] KVM: MMU: abstract spte write-protect Date: Sat, 21 Apr 2012 11:24:54 +0800 Message-ID: <4F922886.5060807@gmail.com> References: <4F911B74.4040305@linux.vnet.ibm.com> <4F911BAB.6000206@linux.vnet.ibm.com> <20120420213319.GA13817@amt.cnet> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Cc: Xiao Guangrong , Avi Kivity , LKML , KVM To: Marcelo Tosatti Return-path: In-Reply-To: <20120420213319.GA13817@amt.cnet> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org On 04/21/2012 05:33 AM, Marcelo Tosatti wrote: >> static bool >> __rmap_write_protect(struct kvm *kvm, unsigned long *rmapp, int level) >> { >> @@ -1050,24 +1078,13 @@ __rmap_write_protect(struct kvm *kvm, unsigned long *rmapp, int level) >> >> for (sptep = rmap_get_first(*rmapp, &iter); sptep;) { >> BUG_ON(!(*sptep & PT_PRESENT_MASK)); >> - rmap_printk("rmap_write_protect: spte %p %llx\n", sptep, *sptep); >> - >> - if (!is_writable_pte(*sptep)) { >> - sptep = rmap_get_next(&iter); >> - continue; >> - } >> - >> - if (level == PT_PAGE_TABLE_LEVEL) { >> - mmu_spte_update(sptep, *sptep & ~PT_WRITABLE_MASK); >> - sptep = rmap_get_next(&iter); >> - } else { >> - BUG_ON(!is_large_pte(*sptep)); >> - drop_spte(kvm, sptep); >> - --kvm->stat.lpages; > > It is preferable to remove all large sptes including read-only ones, the It can cause page faults even if read memory on these large sptse. Actually, Avi suggested that make large writable spte to be readonly (not dropped) on this path. > current behaviour, then to verify that no read->write transition can > occur in fault paths (fault paths which are increasing in number). Yes, the small spte also has issue (find a write-protected spte in fault paths). Later, the second part of this patchset will introduce rmap.WRITE_PROTECTED bit, then we can do the fast check before calling fast page fault.