Re: [virtio-comment] Re: [RFC] ivshmem v2: Shared memory device specification

[Jan Kiszka <jan.kiszka@siemens.com>]

On 27.07.20 15:56, Michael S. Tsirkin wrote:
> On Mon, Jul 27, 2020 at 03:39:32PM +0200, Jan Kiszka wrote:
>> On 27.07.20 15:20, Michael S. Tsirkin wrote:
>>> On Mon, May 25, 2020 at 09:58:28AM +0200, Jan Kiszka wrote:
>>>> #### Vendor Specific Capability (ID 09h)
>>>>
>>>> This capability must always be present.
>>>>
>>>> | Offset | Register            | Content                                        |
>>>> |-------:|:--------------------|:-----------------------------------------------|
>>>> |    00h | ID                  | 09h                                            |
>>>> |    01h | Next Capability     | Pointer to next capability or 00h              |
>>>> |    02h | Length              | 20h if Base Address is present, 18h otherwise  |
>>>> |    03h | Privileged Control  | Bit 0 (read/write): one-shot interrupt mode    |
>>>> |        |                     | Bits 1-7: Reserved (0 on read, writes ignored) |
>>>> |    04h | State Table Size    | 32-bit size of read-only State Table           |
>>>> |    08h | R/W Section Size    | 64-bit size of common read/write section       |
>>>> |    10h | Output Section Size | 64-bit size of output sections                 |
>>>> |    18h | Base Address        | optional: 64-bit base address of shared memory |
>>>>
>>>> All registers are read-only. Writes are ignored, except to bit 0 of
>>>> the Privileged Control register.
>>>
>>>
>>> Is there value in making this follow the virtio vendor-specific
>>> capability format? That will cost several extra bytes - do you envision
>>> having many of these in the config space?
>>
>> Of course, this could be modeled with via virtio_pci_cap as well. Would add
>> 12 unused by bytes and one type byte. If it helps to make the device look
>> more virtio'ish, but I'm afraid there are more differences at PCI level.
> 
> I guess it will be useful if we ever find it handy to make an ivshmem
> device also be a virtio device. Can't say why yet but if we don't care
> it vaguely seems kind of like a good idea. I guess it will also be handy
> if you ever need another vendor specific cap: you already get a way to
> identify it without breaking drivers.
> 

I can look into that. Those 12 wasted bytes are a bit ugly, but so far 
we are not short on config space, even in the non-extended range.

More problematic is that the existing specification of virtio_pci_cap 
assumes that this describes a structure in a PCI resource, rather than 
even being that data itself, and even a register (privileged control).

Would it be possible to split the types into two ranges, one for the 
existing structure, one for others - like ivshmem - that will only share 
the cfg_type field?

> 
>> I do not see a use case for having multiple of those caps above per device.
>> If someone comes around with a valid use case for having multiple,
>> non-consequitive shared memory regions for one device, we would need to add
>> registers for them. But that would also only work for non-BAR regions due to
>> limited BARs.
> 
> 
> OK, I guess this answers the below too.
> 
>>> Also, do we want to define an extended capability format in case this
>>> is a pci extended capability?
>>>
>>
>> What would be the practical benefit? Do you see PCIe caps that could become
>> useful in virtual setups?
> 
> So if we ever have a huge number of these caps, PCIe allows many more
> caps.
> 
>> We don't do that for regular virtio devices
>> either, do we?
> 
> We don't, there's a small number of these so we don't run out of config
> space.

Right. But then it would not a be a problem to add PCIe (right before 
adding it becomes impossible) and push new caps into the extended space. 
And all that without breaking existing drivers. It's just a cap, and the 
spec so far does not state that there must be no other cap, neither in 
current virtio nor this ivshmem device.

Jan

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