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Re: [Xen-devel] [PATCH 2/2] x86: explicitly disallow guest access to PPIN


  • To: Jan Beulich <jbeulich@xxxxxxxx>
  • From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  • Date: Fri, 1 Nov 2019 18:35:13 +0000
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  • Cc: "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
  • Delivery-date: Fri, 01 Nov 2019 18:35:19 +0000
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On 30/10/2019 12:02, Jan Beulich wrote:
> On 30.10.2019 12:43, Andrew Cooper wrote:
>> On 30/10/2019 10:39, Jan Beulich wrote:
>>> To fulfill the "protected" in its name, don't let the real hardware
>>> values "shine through". Report a control register value expressing this.
>>>
>>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>>> ---
>>> TBD: Do we want to permit Dom0 access?
>> I would recommend reordering the two patches and putting this one first
>> (along with the enumeration details, along with a pair of feature
>> strings in xen-cpuid).  This patch at least wants backporting.
> Well, the reason for this ordering is because this way Dom0
> doesn't transiently lose all access.

Nothing pre-existing can be used reliably by dom0 because of the
raz/write-discard behaviour.

I wouldn't complicate patch ordering because of this.

>
> As to xen-cpuid - I admit I simply forgot to update it, largely
> due to there not being any CPUID bit on the Intel side. That part
> would obviously live in whichever patch we elect to be first.
>
>> This would be far more simple if we don't permit dom0 access.  Yes, it
>> shares platform responsibility with Xen, but it also can't do anything
>> more with the value than Xen can, which is to simply print it out for #MCEs.
> Okay, then let's not expose it. I'll drop the TBD.

I'll re-review with dom0 access in mind, but start a new thread.

~Andrew

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