From mboxrd@z Thu Jan 1 00:00:00 1970 From: Marcelo Tosatti Subject: Re: qemu/hw/device-assignment: questions about msix_table_page Date: Tue, 5 May 2009 07:49:10 -0300 Message-ID: <20090505104910.GA4038@amt.cnet> References: <20090427104117.GB29082@redhat.com> <200904272203.59909.sheng@linux.intel.com> <20090427141504.GC2504@redhat.com> <200904272230.18253.sheng@linux.intel.com> <20090505095136.GA12797@redhat.com> <20090505101945.GA11426@amt.cnet> <20090505103450.GB15418@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Sheng Yang , Avi Kivity , kvm@vger.kernel.org To: "Michael S. Tsirkin" Return-path: Received: from mx2.redhat.com ([66.187.237.31]:53225 "EHLO mx2.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755208AbZEEKtY (ORCPT ); Tue, 5 May 2009 06:49:24 -0400 Content-Disposition: inline In-Reply-To: <20090505103450.GB15418@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: On Tue, May 05, 2009 at 01:34:50PM +0300, Michael S. Tsirkin wrote: > On Tue, May 05, 2009 at 07:19:45AM -0300, Marcelo Tosatti wrote: > > On Tue, May 05, 2009 at 12:51:36PM +0300, Michael S. Tsirkin wrote: > > > On Mon, Apr 27, 2009 at 10:30:17PM +0800, Sheng Yang wrote: > > > > > > > > If guest can write to the real device MSI-X table directly, it would > > > > > > > > cause chaos on interrupt delivery, for what guest see is totally > > > > > > > > different with what's host see... > > > > > > > > > > > > > > Obviously. > > > > > > > > > > > > > > Thanks, > > > > > > > > > > What's the reason that this page is unmapped from the qemu memory space? > > > Specifically what do these lines do: > > > int offset = r_dev->msix_table_addr - real_region->base_addr; > > > ret = munmap(region->u.r_virtbase + offset, TARGET_PAGE_SIZE); > > > > I believe this allows accesses to this page (the MSI-X table), which > > is part of the guest address space (through kvm memory slots), to be > > trapped by qemu. > > > > Since there is no actual page in this guest address, KVM treats accesses > > as MMIO and forwards them to QEMU. > > > > > > I thought about this too. > But why is this necessary for assigned MSI-X but not for emulated devices such as > e.g. e1000? All e1000 does seems to be cpu_register_physical_memory ... Because there is no registered (kvm) memory slot for the range which e1000 registers its MMIO? Not sure about the address of the MSI-X table page, but you could achieve the same effect by splitting the slot which it lives in two, with a 1 page hole between them. BTW this is why you can't map the MSI-X table page directly, you want accesses to be trapped.