Hi Rob, Scott, On 06/05/2015 11:14 PM, Scott Wood wrote: > On Fri, 2015-06-05 at 13:05 -0500, Rob Herring wrote: >> On Fri, Jun 5, 2015 at 10:06 AM, Eric Auger <eric.au...@linaro.org> >> wrote: >>> In situations where the userspace driver is stopped abnormally and >>> the >>> VFIO platform device is released, the assigned HW device currently >>> is >>> left running. As a consequence the HW device might continue >>> issuing IRQs >>> and performing DMA accesses. >>> >>> On release, no physical IRQ handler is setup anymore. Also the DMA >>> buffers >>> are unmapped leading to IOMMU aborts. So there is no serious >>> consequence. >>> >>> However when assigning that HW device again to another userspace >>> driver, >>> this latter might face some unexpected IRQs and DMA accesses, >>> which are >>> the result of the previous assignment. >> >> In general, shouldn't it just be a requirement that the drivers >> handle >> this condition. You have the same problem with firmware/bootloaders >> leaving h/w not in reset state or kexec'ing to a new kernel. > > It's not the same situation. Firmware may leave HW in a non-reset > state but it must not leave the HW doing DMA; there's nothing the OS > could do about that as the OS could get corrupted before the driver > has a chance to run (this is not fun to debug). Leaving interrupts > potentially asserted would be bad as well, especially if the interrupt > is shared. > > Likewise, with normal kexec drivers are supposed to quiesce the > hardware first -- and with kdump, the affected DMA buffers are never > reused. > > In order for the driver to handle this, it would need to reset/quiesce > the device itself before enabling an IOMMU mapping. How would that > work for virtualization scenarios where the guest does not see any > IOMMU, and all vfio mappings are handled by QEMU or equivalent?
This is also my understanding. In a KVM virtualization use case, the guest potentially could be corrupted by previously set DMA accesses before getting the chance to stop DMA/IRQs. Thanks for your interest. Best Regards Eric > > -Scott > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
