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RE: [Xen-devel] [PATCH] passthru: Fix pci bar remapping for passthru dev

To: Wei Wang2 <wei.wang2@xxxxxxx>
Subject: RE: [Xen-devel] [PATCH] passthru: Fix pci bar remapping for passthru devices
From: "Jiang, Yunhong" <yunhong.jiang@xxxxxxxxx>
Date: Mon, 20 Jul 2009 23:48:46 +0800
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Thread-topic: [Xen-devel] [PATCH] passthru: Fix pci bar remapping for passthru devices
Wei Wang2 wrote:
> Hi Yunhong
> My testing shows that Linux guests will try to probe pci bar with
> MMIO being enabled. When I assign a broadcom NIC with 32MB MMIO a
> Linux guest, guest will hang after remapping a guest address

Yes, I remember I saw this bug in kernel before.

I suspect the issue here is the local APIC address, which should not be 
intercepted before MMIO/RAM address in native. i.e. the p2m table should not 
cover the local APIC address as RAM, but I think your change in the qemu side 
is more straightforward (otherwise, we may need consider IOAPIC, HPET etc, 
which is complex).

One thing left is, why it will hang, after all, guest will try to restore the 
BAR address later, and at that time, the local apic access can be intercepted 


> "0xfe000000" to physical mmio. However, windows and BSD guests do not
> have this issue. They alway probe mmio size after disabling mmio.
> Thanks,
> Wei
> On Monday 20 July 2009 16:26:00 Jiang, Yunhong wrote:
>> I assume guest should disable the MMIO in PCI_COMMAND before writing
>> all "1"s to bar register. Otherwise, what will happen on native if
>> guest try to access 0xFFFFFFF0? And if we do update the P2M, will it
>> cause trouble to Xen HV? 
>> Thanks
>> Yunhong Jiang
>> xen-devel-bounces@xxxxxxxxxxxxxxxxxxx wrote:
>>> Hi,
>>> When guest code tries to get the block size of mmio, it will write
>>> all "1"s into pci bar register and then qemu will return all "0"s
>>> to the don't care bits in the emulated bar register to indicate the
>>> block size 
>>> to guest code.
>>> In this case, we should not create p2m mapping in
>>> pt_bar_reg_write() and
>>> pt_exp_rom_bar_reg_write(). Attached patch fixes this issue,
>>> additional comment can be found in the patch.
>>> Thanks,
>>> Wei
>>> Signed-off-by: Wei Wang <wei.wang2@xxxxxxx>
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