Alex Williamson wrote:
On Wed, 2009-05-13 at 08:33 -0600, Alex Williamson wrote:
On Wed, 2009-05-13 at 08:15 -0600, Alex Williamson wrote:
On Wed, 2009-05-13 at 16:55 +0300, Michael S. Tsirkin wrote:
Very surprising: I haven't seen any driver disable MSI expect on device
destructor path. Is this a linux guest?
Yes, Debian 2.6.26 kernel. I'll check it it behaves the same on newer
upstream kernels and try to figure out why it's doing it.
Updating the guest to 2.6.29 seems to fix the interrupt toggling. So
it's either something in older kernels or something debian introduced,
but that seems unlikely.
For the curious, this was fixed prior to 2.6.27-rc1 by this:
commit ce6fce4295ba727b36fdc73040e444bd1aae64cd
Author: Matthew Wilcox
Date: Fri Jul 25 15:42:58 2008 -0600
PCI MSI: Don't disable MSIs if the mask bit isn't supported
David Vrabel has a device which generates an interrupt storm on the INTx
pin if we disable MSI interrupts altogether. Masking interrupts is only
a performance optimisation, so we can ignore the request to mask the
interrupt.
It looks like without the maskbit attribute on MSI, the default way to
mask an MSI interrupt was to toggle the MSI enable bit. This was
introduced in 58e0543e8f355b32f0778a18858b255adb7402ae, so it's lifespan
was probably 2.6.21 - 2.6.26.
On the other hand, if the host device supports this maskbit attribute,
we might want to support it. I'm not sure exactly how though.
If we trap msi entry writes, we're inviting the guest to exit every time
it wants to disable interrupts. If we don't, we're inviting spurious
interrupts, which will cause unwanted exits and injections.
Maybe we ought to let the guest write to the msi tables without
trapping, and in the injection logic do something like
msi_entry = *msi_entry_ptr;
mb();
if (msi_entry != msi->last_msi_entry)
msi_reconfigure();
if (msi_enabled(msi_entry))
insert_the_needle();
--
Do not meddle in the internals of kernels, for they are subtle and quick to panic.
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