linux-kernel - Re: [PATCH v3 0/3] uio: add MSI/MSI-X support to uio_pci

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Date:	Tue, 6 Oct 2015 11:37:59 +0300
From:	Vlad Zolotarov <vladz@...udius-systems.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	linux-kernel@...r.kernel.org, hjk@...sjkoch.de, corbet@....net,
	gregkh@...uxfoundation.org, bruce.richardson@...el.com,
	avi@...udius-systems.com, gleb@...udius-systems.com,
	stephen@...workplumber.org, alexander.duyck@...il.com
Subject: Re: [PATCH v3 0/3] uio: add MSI/MSI-X support to uio_pci_generic
 driver



On 10/05/15 22:50, Michael S. Tsirkin wrote:
> On Sun, Oct 04, 2015 at 11:43:15PM +0300, Vlad Zolotarov wrote:
>> This series add support for MSI and MSI-X interrupts to uio_pci_generic driver.
>>   
>> Currently uio_pci_generic supports only legacy INT#x interrupts source. However
>> there are situations when this is not enough, for instance SR-IOV VF devices that
>> simply don't have INT#x capability. For such devices uio_pci_generic will simply
>> fail (more specifically probe() will fail).
>>   
>> When IOMMU is either not available (e.g. Amazon EC2) or not acceptable due to performance
>> overhead and thus VFIO is not an option
>> users that develop user-space drivers are left
>> without any option but to develop some proprietary UIO drivers (e.g. igb_uio driver in Intel's
>> DPDK) just to be able to use UIO infrastructure.
>>   
>> This series provides a generic solution for this problem while preserving the original behaviour
>> for devices for which the original uio_pci_generic had worked before (i.e. INT#x will be used by default).
> What is missing here is that drivers using uio_pci_generic generally
> poke at config and BAR sysfs files of the device.
>
> We can not stop them without breaking existing users, but this means
> that we can't enable bus mastering and MSI/MSI-X blindly: userspace
> bugs will corrupt the MSI-X table and/or MSi/MSI-X capability,
> and cause device to overwrite random addresses, corrupting kernel
> memory.
>
> Your solution seems to be a warning in dmesg and tainting the
> kernel, but that's not enough.
>
> You need to add infrastructure to prevent this.

Bus mastering is easily enabled from the user space (taken from DPDK code):

static int
pci_uio_set_bus_master(int dev_fd)
{
	uint16_t reg;
	int ret;

	ret = pread(dev_fd, &reg, sizeof(reg), PCI_COMMAND);
	if (ret != sizeof(reg)) {
		RTE_LOG(ERR, EAL,
			"Cannot read command from PCI config space!\n");
		return -1;
	}

	/* return if bus mastering is already on */
	if (reg & PCI_COMMAND_MASTER)
		return 0;

	reg |= PCI_COMMAND_MASTER;

	ret = pwrite(dev_fd, &reg, sizeof(reg), PCI_COMMAND);
	if (ret != sizeof(reg)) {
		RTE_LOG(ERR, EAL,
			"Cannot write command to PCI config space!\n");
		return -1;
	}

	return 0;
}

So, this is a non-issue. ;)

>
> VFIO has some code to do this, but it's not bound by existing UIO API so it
> simply fails the mmap.  We want I think existing applications to work,
> so I suspect we need to make a hole there (probably map a zero page in
> case apps want to read it, and maybe even set it up for COW in case they
> tweak the PBA which sometimes happens to be in the same page).
>
> Your patches also seem to add in eventfd and mmap capabilities which
> seems to be orthogonal. They are there in VFIO which I'm guessing is the
> real reason you do it.
>
> So, what you are trying to do might be closer to extending VFIO which
> already has a bunch of checks like that.  Yes, it also wants to program
> the IOMMU.  So maybe do it with a separate device that can be root-only,
> so unpriveledged users can't abuse it.
>
> You should Cc, and talk to the VFIO maintainer.
>
>
>
>> New in v3:
>>     - Add __iomem qualifier to temp buffer receiving ioremap value.
>>
>> New in v2:
>>     - Added #include <linux/uaccess.h> to uio_pci_generic.c
>>
>> Vlad Zolotarov (3):
>>    uio: add ioctl support
>>    uio_pci_generic: add MSI/MSI-X support
>>    Documentation: update uio-howto
>>
>>   Documentation/DocBook/uio-howto.tmpl |  29 ++-
>>   drivers/uio/uio.c                    |  15 ++
>>   drivers/uio/uio_pci_generic.c        | 410 +++++++++++++++++++++++++++++++++--
>>   include/linux/uio_driver.h           |   3 +
>>   include/linux/uio_pci_generic.h      |  36 +++
>>   5 files changed, 467 insertions(+), 26 deletions(-)
>>   create mode 100644 include/linux/uio_pci_generic.h
>>
>> -- 
>> 2.1.0

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