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    SubjectRe: [PATCH v3] xen-pciback: Consider INTx disabled when MSI/MSI-X is enabled
    On Fri, Nov 18, 2022 at 04:49:23PM +0100, Marek Marczykowski-Górecki wrote:
    > Linux enables MSI-X before disabling INTx, but keeps MSI-X masked until
    > the table is filled. Then it disables INTx just before clearing MASKALL
    > bit. Currently this approach is rejected by xen-pciback.
    > According to the PCIe spec, device cannot use INTx when MSI/MSI-X is
    > enabled (in other words: enabling MSI/MSI-X implicitly disables INTx).
    >
    > Change the logic to consider INTx disabled if MSI/MSI-X is enabled. This
    > applies to three places:
    > - checking currently enabled interrupts type,
    > - transition to MSI/MSI-X - where INTx would be implicitly disabled,
    > - clearing INTx disable bit - which can be allowed even if MSI/MSI-X is
    > enabled, as device should consider INTx disabled anyway in that case

    Is this last point strictly needed? From the description above it
    seems Linux only cares about enabling MSI(-X) without the disable INTx
    bit set.

    >
    > Fixes: 5e29500eba2a ("xen-pciback: Allow setting PCI_MSIX_FLAGS_MASKALL too")
    > Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
    > ---
    > Changes in v3:
    > - allow clearing INTx regardless of MSI/MSI-X state, to be consistent
    > with enabling MSI/MSI-X
    > Changes in v2:
    > - restructure the patch to consider not only MASKALL bit, but enabling
    > MSI/MSI-X generally, without explicitly disabling INTx first
    > ---
    > drivers/xen/xen-pciback/conf_space.c | 19 +++++++++++------
    > .../xen/xen-pciback/conf_space_capability.c | 3 ++-
    > drivers/xen/xen-pciback/conf_space_header.c | 21 +++----------------
    > 3 files changed, 18 insertions(+), 25 deletions(-)
    >
    > diff --git a/drivers/xen/xen-pciback/conf_space.c b/drivers/xen/xen-pciback/conf_space.c
    > index 059de92aea7d..d47eee6c5143 100644
    > --- a/drivers/xen/xen-pciback/conf_space.c
    > +++ b/drivers/xen/xen-pciback/conf_space.c
    > @@ -288,12 +288,6 @@ int xen_pcibk_get_interrupt_type(struct pci_dev *dev)
    > u16 val;
    > int ret = 0;
    >
    > - err = pci_read_config_word(dev, PCI_COMMAND, &val);
    > - if (err)
    > - return err;
    > - if (!(val & PCI_COMMAND_INTX_DISABLE))
    > - ret |= INTERRUPT_TYPE_INTX;
    > -
    > /*
    > * Do not trust dev->msi(x)_enabled here, as enabling could be done
    > * bypassing the pci_*msi* functions, by the qemu.
    > @@ -316,6 +310,19 @@ int xen_pcibk_get_interrupt_type(struct pci_dev *dev)
    > if (val & PCI_MSIX_FLAGS_ENABLE)
    > ret |= INTERRUPT_TYPE_MSIX;
    > }

    Since we are explicitly hiding INTx now, should we also do something
    about MSI(X) being both enabled at the same time? The spec states:

    "System configuration software sets one of these bits to enable either
    MSI or MSI-X, but never both simultaneously. Behavior is undefined if
    both MSI and MSI-X are enabled simultaneously."

    So finding both MSI and MSI-X enabled likely means something has gone
    very wrong? Likely to be done in a separate change, just realized
    while looking at the patch context.

    > +
    > + /*
    > + * PCIe spec says device cannot use INTx if MSI/MSI-X is enabled,
    > + * so check for INTx only when both are disabled.
    > + */
    > + if (!ret) {
    > + err = pci_read_config_word(dev, PCI_COMMAND, &val);
    > + if (err)
    > + return err;
    > + if (!(val & PCI_COMMAND_INTX_DISABLE))
    > + ret |= INTERRUPT_TYPE_INTX;
    > + }
    > +
    > return ret ?: INTERRUPT_TYPE_NONE;
    > }
    >
    > diff --git a/drivers/xen/xen-pciback/conf_space_capability.c b/drivers/xen/xen-pciback/conf_space_capability.c
    > index 097316a74126..eb4c1af44f5c 100644
    > --- a/drivers/xen/xen-pciback/conf_space_capability.c
    > +++ b/drivers/xen/xen-pciback/conf_space_capability.c
    > @@ -236,10 +236,11 @@ static int msi_msix_flags_write(struct pci_dev *dev, int offset, u16 new_value,
    > return PCIBIOS_SET_FAILED;
    >
    > if (new_value & field_config->enable_bit) {
    > - /* don't allow enabling together with other interrupt types */
    > + /* don't allow enabling together with other interrupt type */

    This comment needs to be adjusted to note that we allow enabling while
    INTx is not disabled in the command register, in order to please
    Linuxes MSI(-X) startup sequence.

    FWIW, another option would be to simply disable INTX here once MSI(-X)
    is attempted to be enabled, won't that avoid having to modify
    xen_pcibk_get_interrupt_type()?

    Thanks, Roger.

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