Xen devel - Aug 2013 - [PATCH v5] interrupts: allow guest to set/clear MSI-X mask bit

Guest needs the ability to enable and disable MSI-X interrupts
by setting the MSI-X control bit, for a passed-through device.
Guest is allowed to write MSI-X mask bit only if Xen *thinks*
that mask is clear (interrupts enabled). If the mask is set by
Xen (interrupts disabled), writes to mask bit by the guest is
ignored.

Currently, a write to MSI-X mask bit by the guest is silently
ignored.

A likely scenario is where we have a 82599 SR-IOV nic passed
through to a guest. From the guest if you do

  ifconfig <ETH_DEV> down
  ifconfig <ETH_DEV> up

the interrupts remain masked. On VF reset, the mask bit is set
by the controller. At this point, Xen is not aware that mask is set.
However, interrupts are enabled by VF driver by clearing the mask
bit by writing directly to BAR3 region containing the MSI-X table.

From dom0, we can verify that
interrupts are being masked using ''xl debug-keys M''.

Initially, guest was allowed to modify MSI-X bit.
Later this behaviour was changed.
See changeset 74c213c506afcd74a8556dd092995fd4dc38b225.

Signed-off-by: Joby Poriyath <joby.poriyath@citrix.com>
---
 xen/arch/x86/hvm/vmsi.c |   60 ++++++++++++++++++++++++++++++++++++-----------
 1 file changed, 46 insertions(+), 14 deletions(-)

diff --git a/xen/arch/x86/hvm/vmsi.c b/xen/arch/x86/hvm/vmsi.c
index 36de312..3cd54734 100644
--- a/xen/arch/x86/hvm/vmsi.c
+++ b/xen/arch/x86/hvm/vmsi.c
@@ -169,6 +169,7 @@ struct msixtbl_entry
         uint32_t msi_ad[3];	/* Shadow of address low, high and data */
     } gentries[MAX_MSIX_ACC_ENTRIES];
     struct rcu_head rcu;
+    const struct pirq *pirq;
 };
 
 static DEFINE_RCU_READ_LOCK(msixtbl_rcu_lock);
@@ -254,6 +255,8 @@ static int msixtbl_write(struct vcpu *v, unsigned long
address,
     void *virt;
     unsigned int nr_entry, index;
     int r = X86EMUL_UNHANDLEABLE;
+    unsigned long flags, orig;
+    struct irq_desc *desc;
 
     if ( len != 4 || (address & 3) )
         return r;
@@ -283,22 +286,49 @@ static int msixtbl_write(struct vcpu *v, unsigned long
address,
     if ( !virt )
         goto out;
 
-    /* Do not allow the mask bit to be changed. */
-#if 0 /* XXX
-       * As the mask bit is the only defined bit in the word, and as the
-       * host MSI-X code doesn''t preserve the other bits anyway, doing
-       * this is pointless. So for now just discard the write (also
-       * saving us from having to determine the matching irq_desc).
-       */
-    spin_lock_irqsave(&desc->lock, flags);
+    desc = pirq_spin_lock_irq_desc(entry->pirq, &flags);
+    if ( !desc )
+        goto out;
+
+    if ( !desc->msi_desc )
+        goto unlock;
+
     orig = readl(virt);
-    val &= ~PCI_MSIX_VECTOR_BITMASK;
-    val |= orig & PCI_MSIX_VECTOR_BITMASK;
+
+    /*
+     * Do not allow guest to modify MSI-X control bit if it is masked 
+     * by Xen. We''ll only handle the case where Xen thinks that
+     * bit is unmasked, but hardware has silently masked the bit
+     * (in case of SR-IOV VF reset, etc). On the other hand, if Xen 
+     * thinks that the bit is masked, but it''s really not, 
+     * we log a warning.
+     */
+    if ( desc->msi_desc->msi_attrib.masked )
+    {
+        if ( !(orig & PCI_MSIX_VECTOR_BITMASK) )
+            printk(XENLOG_WARNING "MSI-X control bit is unmasked
when"
+                   " it is expected to be masked
[%04x:%02x:%02x.%01x]\n",
+                   entry->pdev->seg, entry->pdev->bus,
+                   PCI_SLOT(entry->pdev->devfn), 
+                   PCI_FUNC(entry->pdev->devfn));
+
+        goto unlock;
+    }
+
+    /*
+     * The mask bit is the only defined bit in the word. But we 
+     * ought to preserve the reserved bits. Clearing the reserved 
+     * bits can result in undefined behaviour (see PCI Local Bus
+     * Specification revision 2.3).
+     */
+    val &= PCI_MSIX_VECTOR_BITMASK;
+    val |= (orig & ~PCI_MSIX_VECTOR_BITMASK);
     writel(val, virt);
-    spin_unlock_irqrestore(&desc->lock, flags);
-#endif
 
+unlock:
+    spin_unlock_irqrestore(&desc->lock, flags);
     r = X86EMUL_OKAY;
+
 out:
     rcu_read_unlock(&msixtbl_rcu_lock);
     return r;
@@ -328,7 +358,8 @@ const struct hvm_mmio_handler msixtbl_mmio_handler = {
 static void add_msixtbl_entry(struct domain *d,
                               struct pci_dev *pdev,
                               uint64_t gtable,
-                              struct msixtbl_entry *entry)
+                              struct msixtbl_entry *entry,
+                              struct pirq *pirq)
 {
     u32 len;
 
@@ -342,6 +373,7 @@ static void add_msixtbl_entry(struct domain *d,
     entry->table_len = len;
     entry->pdev = pdev;
     entry->gtable = (unsigned long) gtable;
+    entry->pirq = pirq;
 
     list_add_rcu(&entry->list, &d->arch.hvm_domain.msixtbl_list);
 }
@@ -404,7 +436,7 @@ int msixtbl_pt_register(struct domain *d, struct pirq *pirq,
uint64_t gtable)
 
     entry = new_entry;
     new_entry = NULL;
-    add_msixtbl_entry(d, pdev, gtable, entry);
+    add_msixtbl_entry(d, pdev, gtable, entry, pirq);
 
 found:
     atomic_inc(&entry->refcnt);
-- 
1.7.10.4

>>> On 15.08.13 at 16:03, Joby Poriyath
<joby.poriyath@citrix.com> wrote:
> Guest needs the ability to enable and disable MSI-X interrupts
> by setting the MSI-X control bit, for a passed-through device.
> Guest is allowed to write MSI-X mask bit only if Xen *thinks*
> that mask is clear (interrupts enabled). If the mask is set by
> Xen (interrupts disabled), writes to mask bit by the guest is
> ignored.
> 
> Currently, a write to MSI-X mask bit by the guest is silently
> ignored.
> 
> A likely scenario is where we have a 82599 SR-IOV nic passed
> through to a guest. From the guest if you do
> 
>   ifconfig <ETH_DEV> down
>   ifconfig <ETH_DEV> up
> 
> the interrupts remain masked. On VF reset, the mask bit is set
> by the controller. At this point, Xen is not aware that mask is set.
> However, interrupts are enabled by VF driver by clearing the mask
> bit by writing directly to BAR3 region containing the MSI-X table.
> 
> From dom0, we can verify that
> interrupts are being masked using ''xl debug-keys M''.
> 
> Initially, guest was allowed to modify MSI-X bit.
> Later this behaviour was changed.
> See changeset 74c213c506afcd74a8556dd092995fd4dc38b225.
> 
> Signed-off-by: Joby Poriyath <joby.poriyath@citrix.com>
> ---
You should start getting used to describe the changes compared
to at least the most recent previous revision here.
> @@ -328,7 +358,8 @@ const struct hvm_mmio_handler msixtbl_mmio_handler = {
>  static void add_msixtbl_entry(struct domain *d,
>                                struct pci_dev *pdev,
>                                uint64_t gtable,
> -                              struct msixtbl_entry *entry)
> +                              struct msixtbl_entry *entry,
> +                              struct pirq *pirq)
Sill missing the const here (this is where Andrew had pointed it out
initially, the other change above just is a necessary consequence).

Jan

On Thu, Aug 15, 2013 at 03:30:52PM +0100, Jan Beulich
wrote:
> >>> On 15.08.13 at 16:03, Joby Poriyath
<joby.poriyath@citrix.com> wrote:
> > Guest needs the ability to enable and disable MSI-X interrupts
> > by setting the MSI-X control bit, for a passed-through device.
> > Guest is allowed to write MSI-X mask bit only if Xen *thinks*
> > that mask is clear (interrupts enabled). If the mask is set by
> > Xen (interrupts disabled), writes to mask bit by the guest is
> > ignored.
> > 
> > Currently, a write to MSI-X mask bit by the guest is silently
> > ignored.
> > 
> > A likely scenario is where we have a 82599 SR-IOV nic passed
> > through to a guest. From the guest if you do
> > 
> >   ifconfig <ETH_DEV> down
> >   ifconfig <ETH_DEV> up
> > 
> > the interrupts remain masked. On VF reset, the mask bit is set
> > by the controller. At this point, Xen is not aware that mask is set.
> > However, interrupts are enabled by VF driver by clearing the mask
> > bit by writing directly to BAR3 region containing the MSI-X table.
> > 
> > From dom0, we can verify that
> > interrupts are being masked using ''xl debug-keys M''.
> > 
> > Initially, guest was allowed to modify MSI-X bit.
> > Later this behaviour was changed.
> > See changeset 74c213c506afcd74a8556dd092995fd4dc38b225.
> > 
> > Signed-off-by: Joby Poriyath <joby.poriyath@citrix.com>
> > ---
> 
> You should start getting used to describe the changes compared
> to at least the most recent previous revision here.
I''ll do this for the next version of the patch.
> 
> > @@ -328,7 +358,8 @@ const struct hvm_mmio_handler msixtbl_mmio_handler
= {
> >  static void add_msixtbl_entry(struct domain *d,
> >                                struct pci_dev *pdev,
> >                                uint64_t gtable,
> > -                              struct msixtbl_entry *entry)
> > +                              struct msixtbl_entry *entry,
> > +                              struct pirq *pirq)
> 
> Sill missing the const here (this is where Andrew had pointed it out
> initially, the other change above just is a necessary consequence).
> 
Somehow the compiler didn''t catch the error. I''m on Debian 7.
I''ll send the updated patch.
> Jan
> 
Joby