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

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

On Mon, Oct 16, 2023 at 03:04:36PM +0200, Marek Marczykowski-Górecki wrote:
> On Mon, Oct 16, 2023 at 11:05:10AM +0200, Roger Pau Monné wrote:
> > 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.
> 
> I'm not sure, but it seems logical to have it symmetric.

I don't have a strong opinion, but I would rather err on the cautious
side and just leave it more strict unless explicitly required.

> > 
> > > 
> > > 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.
> 
> Pciback try to prevent such situation (that's exactly the point of
> checking the current interrupt type). But if you get into such situation
> somehow anyway (likely bypassing pciback), then pciback will still allow
> to disable one of them, so you can fix the situation (the enforcement of
> "only one type at the time" is done setting the enable bit, but you can still
> clear it).
> 
> If both MSI and MSI-X are enabled xen_pcibk_get_interrupt_type() will
> return both bits set.
> 
> > > +
> > > +	/*
> > > +	 * 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.
> 
> Ok.
> 
> > 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()?
> 
> I would rather avoid implicit changes to other bits, it may lead to hard
> to debug corner cases (in this case, for example, if domU decides to
> disable MSI-X later on, it would be left with INTx disabled too, so no
> interrupts at all).

I see, so a case where MSI(-X) setup fails and Linux simply disables
MSI(-X) without clearing INTx disable because it assumes the bit is
not set (because Linux hasn't set it).  Makes sense.

Thanks, Roger.