Re: [PATCH] x86/AMD: adjust SYSCFG, TOM, etc exposure to deal with running nested

[Jan Beulich <jbeulich@xxxxxxxx>]

On 19.07.2021 11:18, Julien Grall wrote:
> On 13/07/2021 08:22, Jan Beulich wrote:
>> In the original change I neglected to consider the case of us running as
>> L1 under another Xen. In this case we're not Dom0, so the underlying Xen
>> wouldn't permit us access to these MSRs. As an immediate workaround use
>> rdmsr_safe(); I don't view this as the final solution though, as the
>> original problem the earlier change tried to address also applies when
>> running nested. Yet it is then unclear to me how to properly address the
>> issue: We shouldn't generally expose the MSR values, but handing back
>> zero (or effectively any other static value) doesn't look appropriate
>> either.
> 
> IIUC, the unsolved problem is a Linux 3.12 dom0 running on top of the L1 
> Xen. The kernel is quite old (and looks to be unsupported), so are we 
> expecting anyone to build a new stack with a newer Xen and such dom0?
> 
> If the answer is unlikely, then I think it would be fair to keep the 
> limitation until someone comes up with such setup.

I might want to put it differently: If you want to run nested, you
shouldn't be using this old a kernel for your Dom0. You saying "looks
to be unsupported" is, aiui, a statement from upstream perspective,
which distros may have a different view on.

>> Fixes: bfcdaae9c210 ("x86/AMD: expose SYSCFG, TOM, TOM2, and IORRs to Dom0")
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> 
> Reviewed-by: Julien Grall <jgrall@xxxxxxxxxx>

Thanks.

Jan