Re: [PATCH v5 03/21] xen/x86: Generate deep dependencies of features

From: Andrew Cooper <andrew.cooper3@citrix.com>
To: Jan Beulich <JBeulich@suse.com>
Cc: Xen-devel <xen-devel@lists.xen.org>
Subject: Re: [PATCH v5 03/21] xen/x86: Generate deep dependencies of features
Date: Fri, 8 Apr 2016 16:18:58 +0100	[thread overview]
Message-ID: <5707CBE2.3090709@citrix.com> (raw)
In-Reply-To: <5707D99D02000078000E60DF@prv-mh.provo.novell.com>

On 08/04/16 16:17, Jan Beulich wrote:
>>>> On 08.04.16 at 01:36, <andrew.cooper3@citrix.com> wrote:
>> On 08/04/2016 00:18, Jan Beulich wrote:
>>>>>> On 07.04.16 at 13:57, <andrew.cooper3@citrix.com> wrote:
>>>> +    deps = {
>>>> +        # FPU is taken to mean support for the x87 regisers as well as the
>>>> +        # instructions.  MMX is documented to alias the %MM registers over the
>>>> +        # x87 %ST registers in hardware.
>>>> +        FPU: [MMX],
>>>> +
>>>> +        # The PSE36 feature indicates that reserved bits in a PSE superpage
>>>> +        # may be used as extra physical address bits.
>>>> +        PSE: [PSE36],
>>>> +
>>>> +        # Entering Long Mode requires that %CR4.PAE is set.  The NX and PKU
>>>> +        # pagetable bits are only representable in the 64bit PTE format
>>>> +        # offered by PAE.
>>>> +        PAE: [LM, NX, PKU],
>>> PKU is explicitly documented to be available only in IA-32e paging,
>>> so I think it should be listed as a dependent of LM.
>> I am not so sure.  Unlike PCID, it is not declared that setting CR4.PKE
>> will result in a #GP fault out of Long Mode, and the description does
>> say "If CR4.PKE = 0, or if IA-32e paging is not active, the processor
>> does not associate linear addresses with protection keys".
>>
>> This suggests that CR4.PKE can be set out of Long Mode, but all
>> pagewalks will act as if key0 is in use.  PCID on the other hand goes to
>> lengths declaring that you can't set CR4.PCIDE out of Long Mode, nor may
>> you attempt to exit Long Mode if it is set.
>>
>> I do have some Skylake hardware with PKU available in the office.  I
>> will investigate what really happens in this case.  (I am also mindful
>> of the fact that 32bit PV guests could in principle use PKU, at which
>> point the dependency on LM would be counter-productive.)
> With the goal of the series going in soon (today?), wouldn't putting
> in the tighter variant be the better approach, allowing us to relax
> things if we find they can be relaxed?

At this point, yes.  I will make a note when changing.

I need to rebase anyway as there is a textual collision with the xsave
bugfixes from yesterday.

~Andrew

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