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

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

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>


Julien Grall



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