[Xen-devel] Re: [PATCH] AMD IOMMU: Fix an interrupt remapping issue

[Wei Wang2 <wei.wang2@xxxxxxx>]

On Friday 08 April 2011 15:43:57 Jan Beulich wrote:
> >>> On 08.04.11 at 13:35, Wei Wang2 <wei.wang2@xxxxxxx> wrote:
> >
> > Some device could generate bogus interrupts if an IO-APIC RTE and an
> > iommu interrupt remapping entry are not consistent during 2 adjacent
> > 64bits IO-APIC RTE updates. For example, if the 2nd operation updates
> > destination bits in RTE for SATA device and unmask it, in some case, SATA
> > device will assert ioapic pin to generate interrupt immediately using new
> > destination but iommu could still translate it into the old destination,
> > then dom0 would be confused. To fix that, we sync up interrupt remapping
> > entry with IO-APIC IRE on every 32 bits operation and foward IOAPIC RTE
> > updates after interrupt remapping table has been changed.
>
> I don't think this is correct: Without the patch, the filling of ioapic_rte
> takes into account the value already written. Now that you only write
> the value at the end of the function, you should overwrite the
> affected half with "value" immediately before calling
> update_intremap_entry_from_ioapic(). 
Sorry, not quite understand your point. My thought is, no matter dom0 tried to 
updates lower half or upper half of RTE, we always updates interrupt table 
from the lower half. This will keep iommu table strictly identically to RTE. 
The old code has an assumption that both lower half and upper of RTE should 
be updated together. But this might not be always true. If by incident, dom0 
only updates the upper half and we don't sync iommu with it, then the 
destination in RTE and iommu table will be different.

> (Without knowing which half 
> gets written, passing "value" to update_intremap_entry_from_ioapic()
> is pointless, and indeed the function doesn't use that parameter.)
True, I will remove this parameter in update_intremap_entry_from_ioapic(). 
ioapic_rte should have all information.

> Eliminating the double write if reg == rte_lo would also seem desirable
> (and in no case should you write back the old value after having called
> update_intremap_entry_from_ioapic()).

It not a write back, It just finishes IO-APIC RTE writes. After updating 
interrupt remapping table we still have to update RTE. It is just a copy of 
__io_apic_write (maybe I should just call it). Old code updates ioapic 
earlier than interrupt remapping table and sata device might generate 
interrupt right after this, which is not expected.

Thanks,
Wei
> Jan




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