Re: [Xen-devel] [PATCH 1/3] PCI/MSI: Add pci_enable_msi_partial()

[Michael Ellerman <mpe@xxxxxxxxxxxxxx>]

On Wed, 2014-07-02 at 14:22 -0600, Bjorn Helgaas wrote:
> On Tue, Jun 10, 2014 at 03:10:30PM +0200, Alexander Gordeev wrote:
> > There are PCI devices that require a particular value written
> > to the Multiple Message Enable (MME) register while aligned on
> > power of 2 boundary value of actually used MSI vectors 'nvec'
> > is a lesser of that MME value:
> > 
> >     roundup_pow_of_two(nvec) < 'Multiple Message Enable'
> > 
> > However the existing pci_enable_msi_block() interface is not
> > able to configure such devices, since the value written to the
> > MME register is calculated from the number of requested MSIs
> > 'nvec':
> > 
> >     'Multiple Message Enable' = roundup_pow_of_two(nvec)
> 
> For MSI, software learns how many vectors a device requests by reading
> the Multiple Message Capable (MMC) field.  This field is encoded, so a
> device can only request 1, 2, 4, 8, etc., vectors.  It's impossible
> for a device to request 3 vectors; it would have to round up that up
> to a power of two and request 4 vectors.
> 
> Software writes similarly encoded values to MME to tell the device how
> many vectors have been allocated for its use.  For example, it's
> impossible to tell the device that it can use 3 vectors; the OS has to
> round that up and tell the device it can use 4 vectors.
> 
> So if I understand correctly, the point of this series is to take
> advantage of device-specific knowledge, e.g., the device requests 4
> vectors via MMC, but we "know" the device is only capable of using 3.
> Moreover, we tell the device via MME that 4 vectors are available, but
> we've only actually set up 3 of them.
> 
> This makes me uneasy because we're lying to the device, and the device
> is perfectly within spec to use all 4 of those vectors.  If anything
> changes the number of vectors the device uses (new device revision,
> firmware upgrade, etc.), this is liable to break.

It also adds more complexity into the already complex MSI API, across all
architectures, all so a single Intel chipset can save a couple of MSIs. That
seems like the wrong trade off to me.

cheers



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