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

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Date:	Mon, 5 Oct 2015 14:47:18 +0300
From:	Avi Kivity <avi@...lladb.com>
To:	Vlad Zolotarov <vladz@...udius-systems.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 10/05/2015 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:

> +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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