Re: [Xen-devel] [PATCH 1/2] iommu/amd: Fix logic for clearing the IOMMU interrupt bits

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 23.04.13 at 02:53, Suravee Suthikulanit <suravee.suthikulpanit@xxxxxxx> 
>>> wrote:
> On 4/19/2013 3:18 AM, Jan Beulich wrote:
>>>>> On 18.04.13 at 20:41, <suravee.suthikulpanit@xxxxxxx> wrote:
>>> The IOMMU interrupt bits in the IOMMU status registers are
>>> cleared when writing 1.  Therefore, the existing logic which reads
>>> the register, set the bit, and then writing back the values
>>> could accidentally clear certain bits if it has been set.
>>>
>>> The correct logic would just be writing only the value which only
>>> set the interrupt bits, and leave the rest to zeros.
>> So looking through the status register accesses, I would assume
>> that guest_iommu_mmio_write64() is broken too? In that it
>> clearly doesn't implement the RW1C behavior for the interrupt
>> and overflow bits?
> Actually, I'm not quite sure why hvm guest would be writing to IOMMU 
> mmio.  I don't actually see AMD IOMMU is exposed to the hvm guest. 
> However, isn't this function is supposed to be passing on the value 
> intended to be written from guest?  In the sense, if the guest is trying 
> to set the bit to 1, the function will just do what it's been told to 
> write to the register?

This is a virtual IOMMU after all. And hence the emulation code
ought to do what real hardware would in the same situation.

Jan


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