linux-kernel - Re: [PATCH v3 2/3] uio_pci_generic: add MSI/MSI-X support

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Date:	Mon, 5 Oct 2015 14:53:26 +0300
From:	Vlad Zolotarov <vladz@...udius-systems.com>
To:	Avi Kivity <avi@...lladb.com>, Greg KH <gregkh@...uxfoundation.org>
Cc:	linux-kernel@...r.kernel.org, mst@...hat.com, hjk@...sjkoch.de,
	corbet@....net, bruce.richardson@...el.com,
	avi@...udius-systems.com, gleb@...udius-systems.com,
	stephen@...workplumber.org, alexander.duyck@...il.com
Subject: Re: [PATCH v3 2/3] uio_pci_generic: add MSI/MSI-X support



On 10/05/15 14:47, Avi Kivity wrote:
> On 10/05/2015 02:41 PM, Vlad Zolotarov wrote:
>>
>>
>> On 10/05/15 13:57, Greg KH wrote:
>>> On Mon, Oct 05, 2015 at 01:48:39PM +0300, Vlad Zolotarov wrote:
>>>>
>>>> On 10/05/15 10:56, Greg KH wrote:
>>>>> On Mon, Oct 05, 2015 at 10:41:39AM +0300, Vlad Zolotarov wrote:
>>>>>>>> +struct msix_info {
>>>>>>>> +    int num_irqs;
>>>>>>>> +    struct msix_entry *table;
>>>>>>>> +    struct uio_msix_irq_ctx {
>>>>>>>> +        struct eventfd_ctx *trigger;    /* MSI-x vector to 
>>>>>>>> eventfd */
>>>>>>> Why are you using eventfd for msi vectors?  What's the reason for
>>>>>>> needing this?
>>>>>> A small correction - for MSI-X vectors. There may be only one MSI 
>>>>>> vector per
>>>>>> PCI function and if it's used it would use the same interface as 
>>>>>> a legacy
>>>>>> INT#x interrupt uses at the moment.
>>>>>> So, for MSI-X case the reason is that there may be (in most cases 
>>>>>> there will
>>>>>> be) more than one interrupt vector. Thus, as I've explained in a 
>>>>>> PATCH1
>>>>>> thread we need a way to indicated each of them separately. 
>>>>>> eventfd seems
>>>>>> like a good way of doing so. If u have better ideas, pls., share.
>>>>> You need to document what you are doing here, I don't see any
>>>>> explaination for using eventfd at all.
>>>>>
>>>>> And no, I don't know of any other solution as I don't know what 
>>>>> you are
>>>>> trying to do here (hint, the changelog didn't document it...)
>>>>>
>>>>>>> You haven't documented how this api works at all, you are going 
>>>>>>> to have
>>>>>>> to a lot more work to justify this, as this greatly increases the
>>>>>>> complexity of the user/kernel api in unknown ways.
>>>>>> I actually do documented it a bit. Pls., check PATCH3 out.
>>>>> That provided no information at all about how to use the api.
>>>>>
>>>>> If it did, you would see that your api is broken for 32/64bit kernels
>>>>> and will fall over into nasty pieces the first time you try to use it
>>>>> there, which means it hasn't been tested at all :(
>>>> It has been tested of course ;)
>>>> I tested it only in 64 bit environment however where both kernel 
>>>> and user
>>>> space applications were compiled on the same machine with the same 
>>>> compiler
>>>> and it could be that "int" had the same number of bytes both in 
>>>> kernel and
>>>> in user space application. Therefore it worked perfectly - I 
>>>> patched DPDK to
>>>> use the new uio_pci_generic MSI-X API to test this and I have 
>>>> verified that
>>>> all 3 interrupt modes work: MSI-X with SR-IOV VF device in Amazon 
>>>> EC2 guest
>>>> and INT#x and MSI with a PF device on bare metal server.
>>>>
>>>> However I agree using uint32_t for "vec" and "fd" would be much more
>>>> correct.
>>> I don't think file descriptors are __u32 on a 64bit arch, are they?
>>
>> I think they are "int" on all platforms and as far as I know u32 
>> should be enough to contain int on any platform.
>>
>
> You need to make sure structures have the same layout on both 32-bit 
> and 64-bit systems, or you'll have to code compat ioctl translations 
> for them.  The best way to do that is to use __u32 so the sizes are 
> obvious, even for int, and to pad everything to 64 bit:

Sure, but the structure below is not the one that is passed in ioctl() - 
it's an internal uio_pci_generic state and there is nothing to worry about.
The one in question is struct uio_pci_generic_irq_set from 
uio_pci_generic.h:

struct uio_pci_generic_irq_set {
     int vec; /* index of the IRQ to connect to starting from 0 */
     int fd;
};

It should be
struct uio_pci_generic_irq_set {
     __u32 vec; /* index of the IRQ to connect to starting from 0 */
     __u32 fd;
};

instead.


>
>> +struct msix_info { 
>
> +    __u32 num_irqs;
> +    __u32 pad; // so pointer below is aligned to 64-bit on both 
> 32-bit and 64-bit userspace
>>
>> +    struct msix_entry *table;
>> +    struct uio_msix_irq_ctx {
>> +        struct eventfd_ctx *trigger;    /* MSI-x vector to eventfd */
>
>
>>>
>>> And NEVER use the _t types in kernel code,
>>
>> Never meant it - it was for a user space interface. For a kernel it's 
>> u32 of course.
>>
>
> For interfaces, use __u32.  You can't use uint32_t because if someone 
> uses C89 in 2015, they may not have <cstdint.h>.
>

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