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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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