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

[Alexander Gordeev <agordeev@xxxxxxxxxx>]

On Thu, Jul 03, 2014 at 09:20:52AM +0000, David Laight wrote:
> From: Bjorn Helgaas
> > 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.
> ...
> 
> Even if you do that, you ought to write valid interrupt information
> into the 4th slot (maybe replicating one of the earlier interrupts).
> Then, if the device does raise the 'unexpected' interrupt you don't
> get a write to a random kernel location.

I might be missing something, but we are talking of MSI address space
here, aren't we? I am not getting how we could end up with a 'write'
to a random kernel location when a unclaimed MSI vector sent. We could
only expect a spurious interrupt at worst, which is handled and reported.

Anyway, as I described in my reply to Bjorn, this is not a concern IMO.

> Plausibly something similar should be done when a smaller number of
> interrupts is assigned.
> 
>       David
> 

-- 
Regards,
Alexander Gordeev
agordeev@xxxxxxxxxx

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