linux-kernel - Re: [PATCH v2] xen-pciback: Consider INTx disabled when MSI/MSI-X is enabled

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Message-ID: <Y3d1UeCuDIMxmzTA@mail-itl>
Date:   Fri, 18 Nov 2022 13:06:41 +0100
From:   Marek Marczykowski-Górecki 
        <marmarek@...isiblethingslab.com>
To:     Jan Beulich <jbeulich@...e.com>
Cc:     Juergen Gross <jgross@...e.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
        "moderated list:XEN HYPERVISOR INTERFACE" 
        <xen-devel@...ts.xenproject.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] xen-pciback: Consider INTx disabled when MSI/MSI-X is
 enabled

On Fri, Nov 18, 2022 at 08:36:14AM +0100, Jan Beulich wrote:
> On 18.11.2022 03:35, 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.
> 
> Similarly the spec doesn't allow using MSI and MSI-X at the same time.
> Before your change xen_pcibk_get_interrupt_type() is consistent for all
> three forms of interrupt delivery; imo it also wants to be consistent
> after your change. This effectively would mean setting only one bit at
> a time (or using an enum right away), but then the question is what
> order you do the checks in. IOW I think the change to the function is
> wrong.

IIUC the difference is that enabling MSI or MSI-X implicitly disables
INTx, while enabling both MSI and MSI-X is UB. This means that MSI
active and PCI_COMMAND_INTX_DISABLE bit not set means "only MSI is
active" - which the function now properly reports.
Both MSI and MSI-X active at the same time means a bug somewhere else
and the current code allows only to disable one of them in such case. I
could replace this with BUG_ON, or simply assume such bug doesn't exist
and ignore this case, if you prefer.

> Furthermore it looks to me as if you're making msi_msix_flags_write()
> inconsistent with command_write() - you'd now want to also permit
> clearing "INTx disable" when MSI or MSI-X are enabled. Which, I think,
> would simply mean allowing the domain unconditional control of the bit
> (as long as allow_interrupt_control is set of course).

I think your are correct.

> Especially with these further changes I'm afraid at least for now I
> view this as moving in the wrong direction. My view might change in
> particular if the description made more clear what was wrong with the
> original change (476878e4b2be ["xen-pciback: optionally allow interrupt
> enable flag writes"]), or perhaps the discussion having led to the form
> which was committed in the end.

I'm afraid I don't understand why you think it's the wrong direction.
Can you clarify? 

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab

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