Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751503AbaGGT1K (ORCPT ); Mon, 7 Jul 2014 15:27:10 -0400 Received: from mail-qc0-f180.google.com ([209.85.216.180]:35366 "EHLO mail-qc0-f180.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751320AbaGGT1E (ORCPT ); Mon, 7 Jul 2014 15:27:04 -0400 MIME-Version: 1.0 In-Reply-To: <20140704085816.GB12247@dhcp-26-207.brq.redhat.com> References: <4fef62a2e647a7c38e9f2a1ea4244b3506a85e2b.1402405331.git.agordeev@redhat.com> <20140702202201.GA28852@google.com> <063D6719AE5E284EB5DD2968C1650D6D1726BF4E@AcuExch.aculab.com> <20140704085816.GB12247@dhcp-26-207.brq.redhat.com> From: Bjorn Helgaas Date: Mon, 7 Jul 2014 13:26:43 -0600 Message-ID: Subject: Re: [PATCH 1/3] PCI/MSI: Add pci_enable_msi_partial() To: Alexander Gordeev Cc: David Laight , "linux-mips@linux-mips.org" , "linux-s390@vger.kernel.org" , "linux-pci@vger.kernel.org" , "x86@kernel.org" , "linux-doc@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "linux-ide@vger.kernel.org" , "iommu@lists.linux-foundation.org" , "xen-devel@lists.xenproject.org" , "linuxppc-dev@lists.ozlabs.org" Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Jul 4, 2014 at 2:58 AM, Alexander Gordeev wrote: > On Thu, Jul 03, 2014 at 09:20:52AM +0000, David Laight wrote: >> From: Bjorn Helgaas >> > On Tue, Jun 10, 2014 at 03:10:30PM +0200, Alexander Gordeev wrote: >> > > There are PCI devices that require a particular value written >> > > to the Multiple Message Enable (MME) register while aligned on >> > > power of 2 boundary value of actually used MSI vectors 'nvec' >> > > is a lesser of that MME value: >> > > >> > > roundup_pow_of_two(nvec) < 'Multiple Message Enable' >> > > >> > > However the existing pci_enable_msi_block() interface is not >> > > able to configure such devices, since the value written to the >> > > MME register is calculated from the number of requested MSIs >> > > 'nvec': >> > > >> > > 'Multiple Message Enable' = roundup_pow_of_two(nvec) >> > >> > For MSI, software learns how many vectors a device requests by reading >> > the Multiple Message Capable (MMC) field. This field is encoded, so a >> > device can only request 1, 2, 4, 8, etc., vectors. It's impossible >> > for a device to request 3 vectors; it would have to round up that up >> > to a power of two and request 4 vectors. >> > >> > Software writes similarly encoded values to MME to tell the device how >> > many vectors have been allocated for its use. For example, it's >> > impossible to tell the device that it can use 3 vectors; the OS has to >> > round that up and tell the device it can use 4 vectors. >> > >> > So if I understand correctly, the point of this series is to take >> > advantage of device-specific knowledge, e.g., the device requests 4 >> > vectors via MMC, but we "know" the device is only capable of using 3. >> > Moreover, we tell the device via MME that 4 vectors are available, but >> > we've only actually set up 3 of them. >> ... >> >> Even if you do that, you ought to write valid interrupt information >> into the 4th slot (maybe replicating one of the earlier interrupts). >> Then, if the device does raise the 'unexpected' interrupt you don't >> get a write to a random kernel location. > > I might be missing something, but we are talking of MSI address space > here, aren't we? I am not getting how we could end up with a 'write' > to a random kernel location when a unclaimed MSI vector sent. We could > only expect a spurious interrupt at worst, which is handled and reported. Yes, that's how I understand it. With MSI, the OS specifies the a single Message Address, e.g., a LAPIC address, and a single Message Data value, e.g., a vector number that will be written to the LAPIC. The device is permitted to modify some low-order bits of the Message Data to send one of several vector numbers (the MME value tells the device how many bits it can modify). Bottom line, I think a spurious interrupt is the failure we'd expect if a device used more vectors than the OS expects it to. Bjorn -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/