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

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Message-ID: <5613DD57.3000906@cloudius-systems.com>
Date:	Tue, 6 Oct 2015 17:40:23 +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 10/06/15 17:30, Michael S. Tsirkin wrote:
> On Tue, Oct 06, 2015 at 11:37:59AM +0300, Vlad Zolotarov wrote:
>> 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. ;)
> There might be valid reasons for DPDK to do this, e.g. if using VFIO.

Michael, I'm afraid u are missing the main point here - the code above 
destroys all your long arguments. U can't possibly prevent the root-user 
from enabling the device bus mastering. And as me and other people on 
this thread have already mentioned MSI and MSI-X device configuration is 
controlled from the device BAR, thus may not be prevented too.


>
> I'm guessing it doesn't enable MSI though, does it?

Again, enabling MSI is a matter of a trivial patch configuring device 
registers on the device BAR.

>

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