From mboxrd@z Thu Jan 1 00:00:00 1970 From: Lai Jiangshan Subject: Re: [PATCH] kvm: rework remove-write-access for a slot Date: Fri, 04 Jun 2010 16:14:08 +0800 Message-ID: <4C08B5D0.6090104@cn.fujitsu.com> References: <4C061C16.9040208@cn.fujitsu.com> <4C063E01.7040206@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Cc: Marcelo Tosatti , LKML , kvm@vger.kernel.org To: Avi Kivity Return-path: In-Reply-To: <4C063E01.7040206@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org Avi Kivity wrote: > On 06/02/2010 11:53 AM, Lai Jiangshan wrote: >> Current code uses slot_bitmap to find ptes who map a page >> from the memory slot, it is not precise: some ptes in the shadow page >> are not map any page from the memory slot. >> >> This patch uses rmap to find the ptes precisely, and remove >> the unused slot_bitmap. >> >> > > Patch looks good; a couple of comments: > > - We might see a slowdown with !tdp, since we no longer have locality. > Each page will map to an spte in a different page. However, it's still > worth it in my opinion. Yes, this patch hurts the cache since we no longer have locality. And if most pages of the slot are not mapped(rmap_next(kvm, rmapp, NULL)==NULL), this patch will worse than old method I think. This patch do things straightly, precisely. > - I thought of a different approach to write protection: write protect > the L4 sptes, on write fault add write permission to the L4 spte and > write protect the L3 sptes that it points to, etc. This method can use > the slot bitmap to reduce the number of write faults. However we can > reintroduce the slot bitmap if/when we use the method, this shouldn't > block the patch. It is very a good approach and it is blazing fast. I have no time to implement it currently, could you update it into the TODO list? > >> >> +static void rmapp_remove_write_access(struct kvm *kvm, unsigned long >> *rmapp) >> +{ >> + u64 *spte = rmap_next(kvm, rmapp, NULL); >> + >> + while (spte) { >> + /* avoid RMW */ >> + if (is_writable_pte(*spte)) >> + *spte &= ~PT_WRITABLE_MASK; > > Must use an atomic operation here to avoid losing dirty or accessed bit. > Atomic operation is too expensive, I retained the comment "/* avoid RMW */" and wait someone take a good approach for it.