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

[Julien Grall <julien@xxxxxxx>]

Hi Jan,

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.
> 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>

Cheers,

--
Julien Grall