Re: [Xen-devel] [PATCH] vt-d: use two 32-bit writes to update DMAR fault address registers

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 18.09.17 at 10:18, <kevin.tian@xxxxxxxxx> wrote:
>>  From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
>> Sent: Monday, September 11, 2017 6:03 PM
>> 
>> >>> On 11.09.17 at 08:00, <haozhong.zhang@xxxxxxxxx> wrote:
>> > The 64-bit DMAR fault address is composed of two 32 bits registers
>> > DMAR_FEADDR_REG and DMAR_FEUADDR_REG. According to VT-d spec:
>> > "Software is expected to access 32-bit registers as aligned doublewords",
>> > a hypervisor should use two 32-bit writes to DMAR_FEADDR_REG and
>> > DMAR_FEUADDR_REG separately in order to update a 64-bit fault
>> address,
>> > rather than a 64-bit write to DMAR_FEADDR_REG.
>> >
>> > Though I haven't seen any errors caused by such one 64-bit write on
>> > real machines, it's still better to follow the specification.
>> 
>> Any sane chipset should split qword accesses into dword ones if
>> they can't be handled at some layer. Also if you undo something
>> explicitly done by an earlier commit, please quote that commit
>> and say what was wrong. After all Kevin as the VT-d maintainer
>> agreed with the change back then.
> 
> I'm OK with this change.

Hmm, would you mind explaining? You were also okay with the
change in the opposite direction back then, and we've had no
reports of problems.

Jan

Jan


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