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Re: [Qemu-devel] [RFC/RFT PATCH v2 0/3] KVM: Introduce KVM_MEM_UNCACHED

From: Christoffer Dall
Subject: Re: [Qemu-devel] [RFC/RFT PATCH v2 0/3] KVM: Introduce KVM_MEM_UNCACHED
Date: Thu, 14 May 2015 14:24:13 +0200
User-agent: Mutt/1.5.21 (2010-09-15)

On Thu, May 14, 2015 at 02:08:49PM +0200, Paolo Bonzini wrote:
> On 14/05/2015 14:00, Christoffer Dall wrote:
> > So, getting back to my original question.  Is the point then that UEFI
> > must assume (from ACPI/DT) the cache-coherency properties of the PCI
> > controller which exists in hardware on the system you're running on,
> > even for the virtual PCI bus because that will be the semantics for
> > assigned devices?
> > 
> > And in that case, we have no way to distinguish between passthrough
> > devices and virtual devices plugged into the virtual PCI bus?
> Well, we could use the subsystem id.  But it's a hack, and may cause
> incompatibilities with some drivers.  Michael, any ideas?
> > What about the idea of having two virtual PCI buses on your system where
> > one is always cache-coherent and uses for virtual devices, and the other
> > is whatever the hardware is and used for passthrough devices?
> I think that was rejected before.

Do you remember where?  I just remember Catalin mentioning the idea to
me verbally.

Besides the slightly heavy added use of resources etc. it seems like it
would address some of our issues in a good way.

But I'm still not sure why UEFI/Linux currently sees our PCI bus as
being non-coherent when in fact it is and we have no passthrough issues
currently.  Are all PCI controllers always non-coherent for some reason
and therefore we model it as such too?


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