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Re: [Xen-devel] [PATCH v5 5/6] xen/x86: add PHYSDEVOP_msi_control

On 06.08.2019 14:43, Marek Marczykowski-Górecki  wrote:
On Tue, Aug 06, 2019 at 02:05:48PM +0200, Jan Beulich wrote:
On 06.08.2019 12:53, Marek Marczykowski-Górecki  wrote:
On Tue, Aug 06, 2019 at 12:33:39PM +0200, Jan Beulich wrote:
On 06.08.2019 11:46, Marek Marczykowski-Górecki  wrote:
On Tue, Aug 06, 2019 at 07:56:39AM +0000, Jan Beulich wrote:
On 05.08.2019 15:44, Marek Marczykowski-Górecki  wrote:
I'm trying to get it working and it isn't clear to me what should I
check for "INTx is also enabled". I assumed PCI_COMMAND_INTX_DISABLE
bit, but it looks like guest has no control over this bit, even in
permissive mode.  This means enabling MSI(-X) always fails because guest
has no way to set PCI_COMMAND_INTX_DISABLE bit before.

Well, the guest has no control, but in order to enable MSI{,-X} I'd
have expected qemu or the Dom0 kernel to set this bit up front.

qemu would do that, when running in dom0. But in PV stubdomain it talks
to pciback, which filters it out.

Filtering out the guest attempt is fine, but it should then set the
bit while preparing MSI/MSI-X for the guest. (I'm not sure it would
need to do so explicitly, or whether it couldn't rely on code
elsewhere in the kernel doing so.)
Well, I think I've made my position clear: So far we use pci_intx()
only on devices used by Xen itself. I think this should remain to be
that way. Devices in possession of domains (including Dom0) should
be under Dom0's control in this regard.

The thing is, in case of using stubdomain, it's mostly stubdomain
handling it. Especially all the config space interception and applying
logic to it is done by qemu in stubdomain. Do you suggest duplicating /
moving that part to dom0 instead? What would be the point for stubdomain

Nothing should be moved between components if possible (and if placed
sensibly). But isn't stubdom (being a PV DomU) relying on pciback in
Dom0 anyway? And hence doesn't the flow of events include
pci_enable_msi{,x}() invoked by pciback? I'd have expected that to
(also) take care of INTx.

This was discussed in v2 of this series[1], where you also responded.
Relevant part of Roger's email:

     Oh great, that's unfortunate. Both pciback functions end up calling
     into msi_capability_init in the Linux kernel, which does indeed more
     than just toggling the PCI config space enable bit.

     OTOH adding a bypass to pciback so the stubdom is able to write to the
     PCI register in order to toggle the enable bit seems quite clumsy. Not
     to mention that you would be required to update Dom0 kernel in order to
     fix the issue.

     Do you think it makes sense to add a domctl to enable/disable MSI(X)?

     This way the bug could be fixed by just updating Xen (and the

[1] https://lists.xenproject.org/archives/html/xen-devel/2019-01/msg01271.html

Hmm, right. But us having closed a circle this way (as it seems)
suggests to me that bypassing Dom0 may not have been the right
idea here after all (i.e. is, as Roger said, clumsy). Also may I
ask whether the quoted there "so this most likely won’t work
correctly" has actually been proven to not work, and not be
possible to make work?


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