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Re: [Xen-devel] [PATCH] x86: properly gate clearing of PKU feature


  • To: Jan Beulich <jbeulich@xxxxxxxx>
  • From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  • Date: Thu, 29 Aug 2019 14:21:34 +0100
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  • Cc: "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
  • Delivery-date: Thu, 29 Aug 2019 13:21:45 +0000
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On 29/08/2019 14:13, Jan Beulich wrote:
> On 29.08.2019 12:46, Andrew Cooper wrote:
>> On 29/08/2019 10:27, Jan Beulich wrote:
>>> setup_clear_cpu_cap() is __init and hence may not be called post-boot.
>>> Note that opt_pku nevertheless is not getting __initdata added - see
>>> e.g. commit 43fa95ae6a ("mm: make opt_bootscrub non-init").
>>>
>>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>> Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>>
>> However, I'm tempted to suggest that this logic disappears entirely. 
>> Given that we don't support it for PV guests, and can't without taking a
>> CR4 write on toggle_kernel_mode(), all this actually controls is whether
>> the bit finds its way into the HVM max policy.
> Well, if you mean replacing opt_pku by an addition to
> parse_xen_cpuid(), then I'll be happy to do that as a follow-on.
> If there is another direction you've been considering, then
> you'd have to at least outline it.

Well - I did say "disappear entirely".  I don't see it as having, or
liable to gain, any useful purpose.

In reality, it was subsumed into cpuid= by my patch to allow all bits to
be tweak-able.

~Andrew

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