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From: Jan Beulich <jbeulich@suse.com>
To: Julien Grall <julien@xen.org>
Cc: "Andrew Cooper" <andrew.cooper3@citrix.com>,
	"Wei Liu" <wl@xen.org>, "Roger Pau Monné" <roger.pau@citrix.com>,
	"xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>
Subject: Re: [PATCH] x86/AMD: adjust SYSCFG, TOM, etc exposure to deal with running nested
Date: Mon, 19 Jul 2021 11:30:53 +0200	[thread overview]
Message-ID: <7786c4b6-15db-e026-45ee-01f851691cd6@suse.com> (raw)
In-Reply-To: <d8913811-2e90-255b-98c6-44e262e113d3@xen.org>

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@suse.com>
> 
> Reviewed-by: Julien Grall <jgrall@amazon.com>

Thanks.

Jan



      reply	other threads:[~2021-07-19  9:31 UTC|newest]

Thread overview: 3+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-13  7:22 [PATCH] x86/AMD: adjust SYSCFG, TOM, etc exposure to deal with running nested Jan Beulich
2021-07-19  9:18 ` Julien Grall
2021-07-19  9:30   ` Jan Beulich [this message]

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