Re: [Xen-devel] [PATCH v3 10/10] x86/MSI-X: provide hypercall interface for mask-all control

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 11/06/15 09:35, Jan Beulich wrote:
>>>> On 05.06.15 at 13:28, <JBeulich@xxxxxxxx> wrote:
>> Qemu shouldn't be fiddling with this bit directly, as the hypervisor
>> may (and now does) use it for its own purposes. Provide it with a
>> replacement interface, allowing the hypervisor to track host and guest
>> masking intentions independently (clearing the bit only when both want
>> it clear).
> Originally I merely meant to ping the tools side changes here
> (considering that the original issue has been pending for months,
> delayed by various security issues as well as slow turnaround on
> understanding the nature and validity of that original issue, I'd
> _really_ like to see this go in now), but thinking about it once
> again over night I realized that what we do here to allow qemu
> to be fixed would then also be made use of by the kernels
> running pciback: While Dom0 fiddling with the MSI-X mask-all bit
> for its own purposes is at least not a security problem, it doing
> so on behalf of (and directed by) a guest would be as soon as
> the hypervisor side patches making use of that bit went in.
>
> While I continue to be of the opinion that all direct writes to
> interrupt masking bits (MSI-X mask-all, MSI-X per-entry mask,
> MSI per entry mask) outside of the hypervisor are wrong and
> should be eliminated, the scope of the problem now clearly
> going beyond qemu made me reconsider whether we shouldn't,
> as advocated by Stefano, follow the trap-and-emulate route
> instead. This would not only mean adding code to x86's existing
> port CF8/CFC intercepts, but also write-protecting the MMCFG
> pages for all PCI devices being MSI or MSI-X capable, emulating
> writes with inspection / modification of writes to any of the mask
> bits located in PCI config space. (A subsequent optimization to
> this may then be a hypercall to do config space writes,
> eliminating the emulation overhead, accompanied by a bitmap
> indicating which devices' CFG space can be written directly.)
>
> For a (from now on) timely resolution of the original problem I'd
> really appreciate opinions (or alternative suggestions).

A very definite +1 from me.  I have previously suggested as much.

~Andrew

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