From mboxrd@z Thu Jan 1 00:00:00 1970 From: Marcelo Tosatti Subject: Re: Single memory slot Date: Thu, 15 Oct 2009 16:46:16 -0300 Message-ID: <20091015194616.GA11656@amt.cnet> References: <4AD6D037.3060301@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: kvm-devel To: Avi Kivity Return-path: Received: from mx1.redhat.com ([209.132.183.28]:32458 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S935395AbZJOTuh (ORCPT ); Thu, 15 Oct 2009 15:50:37 -0400 Received: from int-mx05.intmail.prod.int.phx2.redhat.com (int-mx05.intmail.prod.int.phx2.redhat.com [10.5.11.18]) by mx1.redhat.com (8.13.8/8.13.8) with ESMTP id n9FJoBGm007075 for ; Thu, 15 Oct 2009 15:50:11 -0400 Content-Disposition: inline In-Reply-To: <4AD6D037.3060301@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: On Thu, Oct 15, 2009 at 04:33:11PM +0900, Avi Kivity wrote: > One way to improve the gfn_to_pfn() memslot search is to register just > one slot. This can only work on 64-bit, since even the smallest guests > need 4GB of physical address space. Apart from speeding up > gfn_to_page(), it would also speed up mmio which must iterate over all > slots, so a lookup cache cannot help. > > This would require quite a bunch of changes: > - modify gfn_to_pfn() to fail gracefully if the page is in the slot but > unmapped (hole handling) > - modify qemu to reserve the guest physical address space > - modify qemu memory allocation to use MAP_FIXED to allocate memory > - some hack for the vga aliases (mmap an fd multiple times?) > - some hack for the vmx-specific pages (e.g. APIC-access page) > > Not sure it's worthwhile, but something to keep in mind if a simple > cache or sort by size is insufficient due to mmio. Downside is you lose the ability to write protect a small slot only (could mprotect(MAP_READ) the desired area but get_log+write_protect must be atomic). Also if you enable dirty log for the large slot largepages are disabled.