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Re: [PATCH] PCI/MSI: Fix masking MSI/MSI-X on Xen PV
- To: David Woodhouse <dwmw2@xxxxxxxxxxxxx>
- From: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Date: Mon, 25 Oct 2021 13:43:55 +0200
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- Cc: Jason Andryuk <jandryuk@xxxxxxxxx>, <josef@xxxxxxxxxxx>, <boris.ostrovsky@xxxxxxxxxx>, <helgaas@xxxxxxxxxx>, <jgross@xxxxxxxx>, <linux-pci@xxxxxxxxxxxxxxx>, <maz@xxxxxxxxxx>, <tglx@xxxxxxxxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Mon, 25 Oct 2021 11:44:14 +0000
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On Mon, Oct 25, 2021 at 08:44:52AM +0100, David Woodhouse wrote:
> On Sun, 2021-10-24 at 21:25 -0400, Jason Andryuk wrote:
> > commit fcacdfbef5a1 ("PCI/MSI: Provide a new set of mask and unmask
> > functions") introduce functions pci_msi_update_mask() and
> > pci_msix_write_vector_ctrl() that is missing checks for
> > pci_msi_ignore_mask that exists in commit 446a98b19fd6 ("PCI/MSI: Use
> > new mask/unmask functions"). The checks are in place at the high level
> > __pci_msi_mask_desc()/__pci_msi_unmask_desc(), but some functions call
> > directly to the helpers.
> >
> > Push the pci_msi_ignore_mask check down to the functions that make
> > the actual writes. This keeps the logic local to the writes that need
> > to be bypassed.
> >
> > With Xen PV, the hypervisor is responsible for masking and unmasking the
> > interrupts, which pci_msi_ignore_mask is used to indicate.
>
> This isn't just for Xen PV; Xen HVM guests let Xen unmask the MSI for
> them too.
It's kind of optional for HVM guests, as it depends on
XENFEAT_hvm_pirqs, which sadly gets unconditionally set for HVM
guests, thus dropping any benefits from having hardware assisted APIC
virtualization or posted interrupts support.
AFAICT pci_msi_ignore_mask is not (correctly) set for HVM guest when
MSI interrupts are routed over event channels.
Regards, Roger.
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