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

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Date:	Wed, 16 Jul 2014 11:01:55 +0200
From:	Vitaly Kuznetsov <vkuznets@...hat.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Cc:	stefano.stabellini@...citrix.com, xen-devel@...ts.xenproject.org,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	David Vrabel <david.vrabel@...rix.com>,
	Andrew Jones <drjones@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC 4/4] xen/pvhvm: Make MSI IRQs work after kexec

Konrad Rzeszutek Wilk <konrad.wilk@...cle.com> 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!

> 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-),

> 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
some cases when we see 'crap' instead of pirq here?

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.

Thanks for your comments again!

>> 
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
>> ---
>>  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
--
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