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Re: [Qemu-devel] Using PCI config space to indicate config location
From: |
Rusty Russell |
Subject: |
Re: [Qemu-devel] Using PCI config space to indicate config location |
Date: |
Tue, 09 Oct 2012 10:26:12 +1030 |
User-agent: |
Notmuch/0.13.2 (http://notmuchmail.org) Emacs/23.3.1 (i686-pc-linux-gnu) |
Anthony Liguori <address@hidden> writes:
> Gerd Hoffmann <address@hidden> writes:
>
>> Hi,
>>
>>>> So we could have for virtio something like this:
>>>>
>>>> Capabilities: [??] virtio-regs:
>>>> legacy: BAR=0 offset=0
>>>> virtio-pci: BAR=1 offset=1000
>>>> virtio-cfg: BAR=1 offset=1800
>>>
>>> This would be a vendor specific PCI capability so lspci wouldn't
>>> automatically know how to parse it.
>>
>> Sure, would need a patch to actually parse+print the cap,
>> /me was just trying to make my point clear in a simple way.
>>
>>>>>> 2) ISTR an argument about mapping the ISR register separately, for
>>>>>> performance, but I can't find a reference to it.
>>>>>
>>>>> I think the rationale is that ISR really needs to be PIO but everything
>>>>> else doesn't. PIO is much faster on x86 because it doesn't require
>>>>> walking page tables or instruction emulation to handle the exit.
>>>>
>>>> Is this still a pressing issue? With MSI-X enabled ISR isn't needed,
>>>> correct? Which would imply that pretty much only old guests without
>>>> MSI-X support need this, and we don't need to worry that much when
>>>> designing something new ...
>>>
>>> It wasn't that long ago that MSI-X wasn't supported.. I think we should
>>> continue to keep ISR as PIO as it is a fast path.
>>
>> No problem if we allow to have both legacy layout and new layout at the
>> same time. Guests can continue to use ISR @ BAR0 in PIO space for
>> existing virtio devices, even in case they want use mmio for other
>> registers -> all fine.
>>
>> New virtio devices can support MSI-X from day one and decide to not
>> expose a legacy layout PIO bar.
>
> I think having BAR1 be an MMIO mirror of the registers + a BAR2 for
> virtio configuration space is probably not that bad of a solution.
Well, we also want to clean up the registers, so how about:
BAR0: legacy, as is. If you access this, don't use the others.
BAR1: new format virtio-pci layout. If you use this, don't use BAR0.
BAR2: virtio-cfg. If you use this, don't use BAR0.
BAR3: ISR. If you use this, don't use BAR0.
I prefer the cases exclusive (ie. use one or the other) as a clear path
to remove the legacy layout; and leaving the ISR in BAR0 leaves us with
an ugly corner case in future (ISR is BAR0 + 19? WTF?).
As to MMIO vs PIO, the BARs are self-describing, so we should explicitly
endorse that and leave it to the devices.
The detection is simple: if BAR1 has non-zero length, it's new-style,
otherwise legacy.
Thoughts?
Rusty.
- Re: [Qemu-devel] Proposal for virtio standardization., Anthony Liguori, 2012/10/04
- [Qemu-devel] Using PCI config space to indicate config location, Rusty Russell, 2012/10/07
- Re: [Qemu-devel] Using PCI config space to indicate config location, Anthony Liguori, 2012/10/08
- Re: [Qemu-devel] Using PCI config space to indicate config location, Gerd Hoffmann, 2012/10/08
- Re: [Qemu-devel] Using PCI config space to indicate config location, Anthony Liguori, 2012/10/08
- Re: [Qemu-devel] Using PCI config space to indicate config location, Gerd Hoffmann, 2012/10/08
- Re: [Qemu-devel] Using PCI config space to indicate config location, Anthony Liguori, 2012/10/08
- Re: [Qemu-devel] Using PCI config space to indicate config location,
Rusty Russell <=
- Re: [Qemu-devel] Using PCI config space to indicate config location, Anthony Liguori, 2012/10/08
- Re: [Qemu-devel] Using PCI config space to indicate config location, Rusty Russell, 2012/10/08
- Re: [Qemu-devel] Using PCI config space to indicate config location, Avi Kivity, 2012/10/09
- Re: [Qemu-devel] Using PCI config space to indicate config location, Anthony Liguori, 2012/10/09
- Re: [Qemu-devel] Using PCI config space to indicate config location, Anthony Liguori, 2012/10/09
- Re: [Qemu-devel] Using PCI config space to indicate config location, Rusty Russell, 2012/10/10
- Re: [Qemu-devel] Using PCI config space to indicate config location, Michael S. Tsirkin, 2012/10/10
- Re: [Qemu-devel] Using PCI config space to indicate config location, Jamie Lokier, 2012/10/09
- Re: [Qemu-devel] Using PCI config space to indicate config location, Rusty Russell, 2012/10/10
- Re: [Qemu-devel] Using PCI config space to indicate config location, Rusty Russell, 2012/10/10