[Bug 220740] Host crash when do PF passthrough to KVM guest with some devices

[bugzilla-daemon@kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=220740

--- Comment #5 from Alex Williamson (alex.l.williamson@gmail.com) ---
I have an X710, but not a system that can reproduce the issue.

Also I need to correct my previous statement after untangling the headers. 
This commit did introduce 8-byte access support for archs including x86_64
where they don't otherwise defined a ioread/write64 support.  This access uses
readq/writeq, where previously we'd use pairs or readl/writel.  The expectation
is that we're more closely matching the access by the guest.

I'm curious how we're getting into this code for an X710 though, mine shows
BARs as:

03:00.0 Ethernet controller: Intel Corporation Ethernet Controller X710 for
10GbE SFP+ (rev 01)
        Region 0: Memory at 380000000000 (64-bit, prefetchable) [size=8M]
        Region 3: Memory at 380001800000 (64-bit, prefetchable) [size=32K]

Those would typically be mapped directly into the KVM address space and not
fault through QEMU to trigger access through this code.  The MSI-X capability
lands in BAR3:

        Capabilities: [70] MSI-X: Enable- Count=129 Masked-
                Vector table: BAR=3 offset=00000000
                PBA: BAR=3 offset=00001000

Ideally the device follows the PCIe recommendation not to place registers in
the same page as the vector and pba tables, not doing so could cause this
access though.  If it were such an access, QEMU could virtualize the MSI-X
tables on a different BAR with the option x-msix-relocation=bar5 (or bar2).

If QEMU were using x-no-mmap=on then we could expect this code would be used,
but that's not specified in the example.

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