Re: [PATCH v3] xen-pciback: Consider INTx disabled when MSI/MSI-X is enabled

["Roger Pau Monné" <roger.pau@citrix.com>]

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.