Re: [PATCH v2] interrupts: allow guest to set and clear MSI-X mask bit

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 23/07/13 11:54, Joby Poriyath wrote:
> Ping...
>
> Could you kindly review this?
>
> Many thanks,
> Joby
>
> On Fri, Jul 19, 2013 at 04:07:37PM +0100, Joby Poriyath wrote:
>> Guest needs the ability to enable and disable MSI-X interrupts
>> by setting the MSI-X control bit. 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.  The the mask bit for the VF is
>> being set by the PF performing a reset (at the request of the VF).
>> 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>

Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

This should be backported to all stable versions (perhaps including 4.1)
as the buggy changeset 74c213c50 was backported in the name of
security/general fixes.

~Andrew

>> ---
>>  xen/arch/x86/hvm/vmsi.c |   32 +++++++++++++++++++-------------
>>  1 file changed, 19 insertions(+), 13 deletions(-)
>>
>> diff --git a/xen/arch/x86/hvm/vmsi.c b/xen/arch/x86/hvm/vmsi.c
>> index 36de312..97d9f93 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;
>> +    struct pirq *pirq;
>>  };
>>  
>>  static DEFINE_RCU_READ_LOCK(msixtbl_rcu_lock);
>> @@ -254,6 +255,9 @@ static int msixtbl_write(struct vcpu *v, unsigned long address,
>>      void *virt;
>>      unsigned int nr_entry, index;
>>      int r = X86EMUL_UNHANDLEABLE;
>> +    unsigned long flags;
>> +    struct irq_desc *desc;
>> +    unsigned long orig;
>>  
>>      if ( len != 4 || (address & 3) )
>>          return r;
>> @@ -283,20 +287,20 @@ 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;
>> +
>> +   /* 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).
>> +    */
>>      orig = readl(virt);
>> -    val &= ~PCI_MSIX_VECTOR_BITMASK;
>> -    val |= orig & PCI_MSIX_VECTOR_BITMASK;
>> +    val &= PCI_MSIX_VECTOR_BITMASK;
>> +    val |= ( orig & ~PCI_MSIX_VECTOR_BITMASK );
>>      writel(val, virt);
>>      spin_unlock_irqrestore(&desc->lock, flags);
>> -#endif
>>  
>>      r = X86EMUL_OKAY;
>>  out:
>> @@ -328,7 +332,8 @@ 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 +347,7 @@ 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 +410,7 @@ 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
>>