linux-kernel - Re: [PATCH] xen-pciback: Consider MSI-X enabled only when MASKALL bit is cleared

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Message-ID: <e606aa11-aeb7-fcaa-d699-380be4adc1ac@citrix.com>
Date:   Thu, 17 Nov 2022 12:28:46 +0000
From:   Andrew Cooper <Andrew.Cooper3@...rix.com>
To:     Marek Marczykowski-Górecki 
        <marmarek@...isiblethingslab.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     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>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] xen-pciback: Consider MSI-X enabled only when MASKALL bit
 is cleared

On 17/11/2022 11:41, 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.
> Allow setting PCI_MSIX_FLAGS_ENABLE while INTx is still enabled as long
> as PCI_MSIX_FLAGS_MASKALL is set too.
>
> Fixes: 5e29500eba2a ("xen-pciback: Allow setting PCI_MSIX_FLAGS_MASKALL too")
> Signed-off-by: Marek Marczykowski-Górecki <marmarek@...isiblethingslab.com>

The PCI spec states that devices are not permitted to use INTx when MSI
or MSI-X is enabled.  The mask status has no legitimate bearing on irq type.

INTx_DISABLE exists as a bodge to mean "INTx not permitted even when
neither MSI nor MSI-X are enabled", and exists because in some case,
transiently disabling MSI is the only safe way to update the descriptor.


I can believe that this change fixes a an issue, but the logic surely
cannot be correct overall.

~Andrew