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[Qemu-devel] Re: [PATCH V6 17/32] pci: 64bit bar support.

From: Michael S. Tsirkin
Subject: [Qemu-devel] Re: [PATCH V6 17/32] pci: 64bit bar support.
Date: Tue, 3 Nov 2009 13:47:56 +0200
User-agent: Mutt/1.5.19 (2009-01-05)

On Tue, Nov 03, 2009 at 12:52:10PM +0900, Isaku Yamahata wrote:
> On Sun, Nov 01, 2009 at 06:07:30PM +0200, Michael S. Tsirkin wrote:
> > On Fri, Oct 30, 2009 at 09:21:11PM +0900, Isaku Yamahata wrote:
> > > implemented pci 64bit bar support.
> > > The tricky bit is pci_update_mapping().
> > > An OS is allowed to set the BAR such that OS can't address the area
> > > pointed by BAR. It doesn't make sense, though.
> > 
> > It might make sense. 32 bit guest can address more than 4G of
> > physical RAM, e.g. using PAE.
> Yes, in that case, guest OS will set bar to be under 36 bit.
> If PAE were supported, target phys address would be 64 bit.
> > Since I think qemu can not support this if target phys address is 32
> > bit, we should declare lack of support for 64 bit addressing on these
> > platforms, by forcing BAR into 32 bit mode, rather than silently failing
> > to map it.
> I don't get your point. And I don't understand the benefit of focing
> BAR into 32 bit mode.
> Real hardware silently maps BAR to address beyond CPU addressable
> area.
> Let's stick to PCI spec as you said before.
> 32 bit guest OS will set 64 bit BAR to be smaller 32bit
> (or 36bit if PAE). That's it.
> Even if 64 bit CPU case, architectally addressable address bit is smaller
> than 64 bit. It's CPU implementation dependent.
> And guest OS sets BAR according to it.
> -- 
> yamahata

IMO, this is not a question of what guest does or does not do.

If qemu is compiled with target phys address size 32 bit, emulated
devices can not support a 64 bit BAR.  Therefore, according to PCI spec,
such devices should declare all BARs as 32 bit.

I think you are right that guests on such systems really do not have a
way to address PCI devices if BAR is set beyond 4G. But pci emulation is
better off not relying on this, IMO. Makes sense?


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