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

[Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>]

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.

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?

Igor

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