linux-kernel - Re: [PATCH v2] xen-pciback: optionally allow interrupt enable flag writes

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Message-ID: <20191219125713.GN25235@mail-itl>
Date:   Thu, 19 Dec 2019 13:57:13 +0100
From:   Marek Marczykowski-Górecki 
        <marmarek@...isiblethingslab.com>
To:     Jan Beulich <jbeulich@...e.com>
Cc:     xen-devel@...ts.xenproject.org,
        Roger Pau Monné <roger.pau@...rix.com>,
        YueHaibing <yuehaibing@...wei.com>,
        Simon Gaiser <simon@...isiblethingslab.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] xen-pciback: optionally allow interrupt enable flag
 writes

On Thu, Dec 19, 2019 at 12:20:24PM +0100, Jan Beulich wrote:
> On 19.12.2019 04:49, Marek Marczykowski-Górecki  wrote:
> > +enum interrupt_type xen_pcibk_get_interrupt_type(struct pci_dev *dev)
> > +{
> > +	int err;
> > +	u16 val;
> > +
> > +	err = pci_read_config_word(dev, PCI_COMMAND, &val);
> > +	if (err)
> > +		return INTERRUPT_TYPE_ERR;
> > +	if (!(val & PCI_COMMAND_INTX_DISABLE))
> > +		return INTERRUPT_TYPE_INTX;
> > +
> > +	/* Do not trust dev->msi(x)_enabled here, as enabling could be done
> > +	 * bypassing the pci_*msi* functions, by the qemu.
> > +	 */
> 
> Judging from this comment, how can you assume only one of the
> three variants is actually enabled? It's against the spec, yes,
> but it's not at all impossible afaict. I think you want the
> return value here to be
> - negative errno values (no need to discard the actual error
>   codes) or
> - a non-negative bitmap indicating which of the interrupt types
>   is/are currently enabled.

Good idea, I'll change that.

> That way ...
> 
> > +static int msi_msix_flags_write(struct pci_dev *dev, int offset, u16 new_value,
> > +				void *data)
> > +{
> > +	int err;
> > +	u16 old_value;
> > +	const struct msi_msix_field_config *field_config = data;
> > +	const struct xen_pcibk_dev_data *dev_data = pci_get_drvdata(dev);
> > +
> > +	if (xen_pcibk_permissive || dev_data->permissive)
> > +		goto write;
> > +
> > +	err = pci_read_config_word(dev, offset, &old_value);
> > +	if (err)
> > +		return err;
> > +
> > +	if (new_value == old_value)
> > +		return 0;
> > +
> > +	if (!dev_data->allow_interrupt_control ||
> > +	    (new_value ^ old_value) & ~field_config->enable_bit)
> > +		return PCIBIOS_SET_FAILED;
> > +
> > +	if (new_value & field_config->enable_bit) {
> > +		/* don't allow enabling together with other interrupt types */
> > +		const enum interrupt_type int_type = xen_pcibk_get_interrupt_type(dev);
> > +		if (int_type == INTERRUPT_TYPE_NONE ||
> > +		    int_type == field_config->int_type)
> 
> ... equality comparisons like this one will actually become safe.
> 
> Jan

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab
A: Because it messes up the order in which people normally read text.
Q: Why is top-posting such a bad thing?

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