Michael S. Tsirkin wrote:
On Sun, May 17, 2009 at 11:47:53PM +0300, Avi Kivity wrote:
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.
Avi, I think you are mixing up MSI and MSI-X. MSI does not have any tables:
all changes go through configuration writes, which AFAIK we always trap.
Isn't that right?
Right.
On the other hand, in MSI-X mask bit is mandatory, not optional
so we'll have to support it for assigned devices at some point.
If we are worried about speed of masking/unmasking MSI-X interrupts for
assigned devices (older kernels used to mask them, recent kernels leave
this to drivers) we will probably need to have MSI-X support in the
kernel, and have kernel examine the mask bit before injecting the
interrupt, just like real devices do.
Yes. Let's actually quantify this though. Several times per second
doesn't qualify.
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();
I don't really understand why do you need the reconfigure
and tracking last msi entry here.
The msi entry can change the vector and delivery mode, no? This way we
can cache some stuff instead of decoding it each time. For example, if
the entry points at a specific vcpu, we can cache the vcpu pointer,
instead of searching for the apic ID.
--
error compiling committee.c: too many arguments to function
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