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    SubjectRe: [PATCH RFC 4/4] xen/pvhvm: Make MSI IRQs work after kexec
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    Konrad Rzeszutek Wilk <konrad.wilk@oracle.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@redhat.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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