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

[Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>]

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?

>> 
>> > 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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