Re: [Xen-devel] [PATCH RESEND 1/3] OvmfPkg/XenSupport: remove usage of prefetchable PCI host bridge aperture

[Anthony PERARD <anthony.perard@xxxxxxxxxx>]

On Thu, Mar 14, 2019 at 07:45:56PM +0000, Igor Druzhinin wrote:
> On 14/03/2019 17:41, Anthony PERARD wrote:
> > Hi,
> > 
> > On Wed, Mar 06, 2019 at 12:40:54PM +0000, Igor Druzhinin wrote:
> >> This aperture doesn't exist in OVMF and trying to use it causes
> > 
> > I'm trying to understand what you mean by writing "doesn't exist in
> > OVMF". Are prefetchable BAR not handled by ScanForRootBridges() ?
> > Or is it the emulation of the config space that isn't correct?
> > Maybe QEMU should lies about a BAR been prefetchable?
> 
> The problem here is: hvmloader places BARs initially disregarding
> prefetchable bit in an arbitrary order because essentially there is only
> 1 aperture for the host bridge in emulated system under Xen (and KVM as
> well). In PcatPciRootBridgeParseBars() we construct apertures for high
> level OVMF code by reading the BAR placement information after
> hvmloader. It often appears that there are prefetchable and
> non-prefetchable BARs coexist with each other and make prefetchable and
> non-prefetchable apertures overlap. This eventually triggers an
> assertion in high level OVMF code because that shouldn't happen.

Thanks for the explanation. Could you add it to the patch description?

> OVMF for KVM is not using prefetchable BAR at all - see
> PciHostBridgeGetRootBridges() in which it passes mNonExistAperture dummy
> object to high level code. I think it's wrong to construct a
> prefetchable aperture for Xen and this code should be removed as it's
> done for QEMU-KVM. Do you think this patch needs to do that?

It would be nice to remove the code that isn't useful so feel free to do
it and/or keep the current patch with the description updated.

Thanks,

-- 
Anthony PERARD

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