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

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

On 09/04/15 09:51, David Vrabel wrote:
> On 08/04/15 20:44, Andrew Cooper wrote:
>> 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().
> Could RMAD be specified as a real table in the future?  Does the
> clobbered name have to start with X to avoid future conflicts?
>
> David

I am not aware of any restrictions imposed by the APCI spec.  Any
clobbered signature is potentially a real table in the future.

This DMAR clobbering was introduced by
83904107a33c9badc34ecdd1f8ca0f9271e5e370 which claims that the dom0 VT-d
driver was capable of playing with the IOMMU(s) while Xen was also using
them.  An alternative approach might be to leave the DMAR table alone
and sprinkle some iomem_deny_access() around to forcibly prevent dom0
from playing.

~Andrew