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Date: Mon, 16 Oct 2023 15:29:57 +0200
From: Roger Pau Monné <roger.pau@...rix.com>
To: Marek Marczykowski-Górecki
<marmarek@...isiblethingslab.com>
Cc: linux-kernel@...r.kernel.org, Juergen Gross <jgross@...e.com>,
Stefano Stabellini <sstabellini@...nel.org>,
Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
Jan Beulich <jbeulich@...e.com>,
"moderated list:XEN HYPERVISOR INTERFACE"
<xen-devel@...ts.xenproject.org>
Subject: Re: [PATCH v3] xen-pciback: Consider INTx disabled when MSI/MSI-X is
enabled
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@...isiblethingslab.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.
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