linux-kernel - Re: [patch 2/8] PCI/MSI: Mask all unused MSI-X entries

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Message-ID: <8735s631hq.wl-maz@kernel.org>
Date:   Thu, 22 Jul 2021 14:46:25 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     "Raj, Ashok" <ashok.raj@...el.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>, linux-pci@...r.kernel.org,
        "David S. Miller" <davem@...emloft.net>,
        Kevin Tian <kevin.tian@...el.com>,
        Ingo Molnar <mingo@...nel.org>, x86@...nel.org
Subject: Re: [patch 2/8] PCI/MSI: Mask all unused MSI-X entries

On Wed, 21 Jul 2021 23:57:55 +0100,
Thomas Gleixner <tglx@...utronix.de> wrote:
> 
> Ashok,
> 
> On Wed, Jul 21 2021 at 15:23, Ashok Raj wrote:
> > On Wed, Jul 21, 2021 at 09:11:28PM +0200, Thomas Gleixner wrote:
> >>  
> >> +		addr = pci_msix_desc_addr(entry);
> >> +		if (addr)
> >> +			entry->masked = readl(addr + PCI_MSIX_ENTRY_VECTOR_CTRL);
> >
> > Silly question:
> > Do we have to read what the HW has to set this entry->masked? Shouldn't
> > this be all masked before we start the setup?
> 
> msix_mask_all() is invoked before the msi descriptors are
> allocated. msi_desc::masked is actually a misnomer because it's not like
> the name suggests a boolean representing the masked state. It's caching
> the content of the PCI_MSIX_ENTRY_VECTOR_CTRL part of the corresponding
> table entry. Right now this is just using bit 0 (the mask bit), but is
> that true forever? So we actually should rename that member to
> vector_ctrl or such.

To follow-up with this forward looking statement, should we only keep
bit 0 when reading PCI_MSIX_ENTRY_VECTOR_CTRL? I.e.:

	entry->masked = (readl(addr + PCI_MSIX_ENTRY_VECTOR_CTRL) &
			 PCI_MSIX_ENTRY_CTRL_MASKBIT);

Or do we want to cache the whole register? In which case I'm all for
the suggesting renaming (though 'masked' is shared with the old-school
multi-MSI).

Otherwise:

Reviewed-by: Marc Zyngier <maz@...nel.org>

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.