On Fri, Jul 4, 2014 at 2:58 AM, Alexander Gordeev <agordeev@xxxxxxxxxx> 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-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
