Re: [PATCH] VTd/dmar: Tweak how the DMAR table is clobbered

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

On 10/04/15 02:23, Tian, Kevin wrote:
>> From: Andrew Cooper [mailto:andrew.cooper3@citrix.com]
>> Sent: Thursday, April 09, 2015 3:45 AM
>>
>> Intead of clobbering DMAR -> XMAR and back, clobber to RMAD instead.
>> This
>> means that changing the signature does not alter the checksum, which allows
>> the clobbering/unclobbering to be peformed atomically and idempotently,
>> which
>> is an advantage on the kexec path which can reenter acpi_dmar_reinstate().
>>
>> Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
>> CC: Yang Zhang <yang.z.zhang@intel.com>
>> CC: Kevin Tian <kevin.tian@intel.com>
> Acked-by: Kevin Tian <kevin.tian@intel.com>
>
> and curious do you observe a real atomic issue in kexec or just catch this
> potential issue when reading code? :-)

I have run over it once in the past, but mainly it is one small thing on
a very long list of tweaks to make the crash path for reliable.

As indicated in the other thread, I think the best direction moving
forwards is to see about positively preventing dom0 having access,
rather than simply hiding the table, but that is a job for another time.

~Andrew