Re: [PATCH RFC 4/4] xen/pvhvm: Make MSI IRQs work after kexec

[Konrad Rzeszutek Wilk]

On Wed, Jul 16, 2014 at 07:20:39PM +0200, Vitaly Kuznetsov wrote:
> Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx> writes:
> 
> > On Wed, Jul 16, 2014 at 11:01:55AM +0200, Vitaly Kuznetsov wrote:
> >> Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx> writes:
> >> 
> >> > On Tue, Jul 15, 2014 at 03:40:40PM +0200, Vitaly Kuznetsov wrote:
> >> >> When kexec was peformed MSI IRQs for passthrough-ed devices were already
> >> >> mapped and we see non-zero pirq extracted from MSI msg. xen_irq_from_pirq()
> >> >> fails as we have no IRQ mapping information for that. Requesting for new
> >> >> mapping with __write_msi_msg() does not result in MSI IRQ being remapped so
> >> >> we don't recieve these IRQs.
> >> >
> >> > receive
> >> >
> >> 
> >> Thanks for your comments!
> >
> > Thank you for quick turnaround with the answers!
> >> 
> >> > How come '__write_msi_msg' does not result in new MSI IRQs?
> >> >
> >> 
> >> Actually that was the hidden question in my RFC :-)
> >> 
> >> Let me describe what I see. When normal boot is performed we have the
> >> following in xen_hvm_setup_msi_irqs():
> >> 
> >> __read_msi_msg()
> >>  pirq -> 0
> >> 
> >> then we allocate new pirq with
> >>  pirq = xen_allocate_pirq_msi()
> >>  pirq -> 54
> >> 
> >> and we have the following mapping:
> >> xen: msi --> pirq=54 --> irq=72
> >> 
> >> in 'xl debug-keys i':
> >> (XEN)    IRQ:  29 affinity:04 vec:b9 type=PCI-MSI status=00000030 in-flight=0 domain-list=7: 54(----),
> >> 
> >> After kexec we see the following:
> >> __read_msi_msg()
> >>  pirq -> 54
> >> 
> >> but as xen_irq_from_pirq() fails we follow the same path allocating new pirq:
> >>  pirq = xen_allocate_pirq_msi()
> >>  pirq -> 55
> >> 
> >> and we have the following mapping:
> >> xen: msi --> pirq=55 --> irq=75
> >> 
> >> However (afaict) mapping in xen wasn't updated:
> >> 
> >> in 'xl debug-keys i':
> >> (XEN)    IRQ:  29 affinity:02 vec:b9 type=PCI-MSI status=00000030 in-flight=0 domain-list=7: 54(--M-),
> >
> > I am wondering if that is related to in QEMU traditional:
> >
> >     qemu-xen-trad: free all the pirqs for msi/msix when driver unloads
> >
> > (which in the upstream QEMU is 1d4fd4f0e2fc5dcae0c60e00cc9af95f52988050)
> >
> > If you have that patch in, is the PIRQ value correctly updated?
> >
> 
> Thanks, that really works! I tested both kexec -e / kdump cases. I'm
> wondering if we although need my commit to workaround non-fixed qemus?

Without your patch on older QEMU's with PCI passthrough we won't get
any more interrupts after we kexec in the guest right?

As in, this issue happens _only_ with PCI passthrough devices that use
MSI or MSI-X?


Still need to get Stefano's view on this.

> 
> >> 
> >> > Is it fair to state that your code ends up reading the MSI IRQ (PIRQ)
> >> > from the device and updating the internal PIRQ<->IRQ code to match
> >> > with the reality?
> >> >
> >> 
> >> Yea, 'always trust the device'.
> >> 
> >> >> 
> >> >> RFC: I wasn't able to understand why commit af42b8d1 which introduced
> >> >> xen_irq_from_pirq() check in xen_hvm_setup_msi_irqs() is checking that instead
> >> >> of checking pirq > 0 as if the mapping was already done (and we have pirq>0 here)
> >> >> we don't need to request for a new pirq. We're loosing existing PIRQ and I'm also
> >> >> not sure when __write_msi_msg() with new PIRQ will result in new mapping.
> >> >
> >> > We don't request a new pirq. We end up returning before we call xen_allocate_pirq_msi.
> >> > At least that is how the commit you mentioned worked.
> >> >
> >> 
> >> I meant to say that in case we have pirq > 0 from __read_msi_msg() but
> >> xen_irq_from_pirq(pirq) fails (kexec-only case?) we always do
> >> xen_allocate_pirq_msi() which brings us new pirq.
> >> 
> >> > In regards to why using 'xen_irq_from_pirq' instead of just checking the PIRQ - is
> >> > that we might be called twice by a buggy driver. As such we want to check
> >> > our PIRQ<->IRQ to figure this out.
> >> 
> >> But if we're called twice we'll see the same pirq, right? Or there are
> >
> > Good point.
> >> some cases when we see 'crap' instead of pirq here?
> >
> > For PCI passthrough devices they will be zero until they are enabled.
> > But I am not sure about the emulated devices, such as e1000 or such, which
> > would also go through this path (I think - do we have MSI devices that
> > we emulate in QEMU?)
> 
> AFAICT emulated e1000 doesn't use MSI (at least with qemu-tradidtional)
> and with my patch series it works after kexec.
> 
> >
> >> 
> >> I think it would be nice to use the same pirq after kexec instead of
> >> allocating a new one even in case we can make remapping work.
> >
> > I concur.
> >
> > Stefano, do you recall why you used xen_irq_from_pirq instead of just
> > trusting the 'pirq' value? Was it to workaround broken QEMU?
> >
> >> 
> >> Thanks for your comments again!
> >> 
> >> >> 
> >> >> Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> >> >> ---
> >> >>  arch/x86/pci/xen.c | 3 +--
> >> >>  1 file changed, 1 insertion(+), 2 deletions(-)
> >> >> 
> >> >> diff --git a/arch/x86/pci/xen.c b/arch/x86/pci/xen.c
> >> >> index 905956f..685e8f1 100644
> >> >> --- a/arch/x86/pci/xen.c
> >> >> +++ b/arch/x86/pci/xen.c
> >> >> @@ -231,8 +231,7 @@ static int xen_hvm_setup_msi_irqs(struct pci_dev *dev, int nvec, int type)
> >> >>  		__read_msi_msg(msidesc, &msg);
> >> >>  		pirq = MSI_ADDR_EXT_DEST_ID(msg.address_hi) |
> >> >>  			((msg.address_lo >> MSI_ADDR_DEST_ID_SHIFT) & 0xff);
> >> >> -		if (msg.data != XEN_PIRQ_MSI_DATA ||
> >> >> -		    xen_irq_from_pirq(pirq) < 0) {
> >> >> +		if (msg.data != XEN_PIRQ_MSI_DATA || pirq <= 0) {
> >> >>  			pirq = xen_allocate_pirq_msi(dev, msidesc);
> >> >>  			if (pirq < 0) {
> >> >>  				irq = -ENODEV;
> >> >> -- 
> >> >> 1.9.3
> >> >> 
> >> 
> >> -- 
> >>   Vitaly
> 
> -- 
>   Vitaly
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