Re: [Xen-devel] [PATCH v3] x86: do not enable global pages when virtualized on AMD hardware

[Jan Beulich <jbeulich@xxxxxxxx>]

On 04.12.2019 17:18, Roger Pau Monné wrote:
> On Wed, Dec 04, 2019 at 05:11:42PM +0100, Jan Beulich wrote:
>> On 04.12.2019 16:12, Roger Pau Monne wrote:
>>> @@ -130,7 +143,7 @@ unsigned long pv_make_cr4(const struct vcpu *v)
>>>       */
>>>      if ( d->arch.pv.pcid )
>>>          cr4 |= X86_CR4_PCIDE;
>>> -    else if ( !d->arch.pv.xpti )
>>> +    else if ( !d->arch.pv.xpti && opt_global_pages )
>>>          cr4 |= X86_CR4_PGE;
>>
>> I'm sorry for noticing this only now, but what about XEN_MINIMAL_CR4,
>> which includes X86_CR4_PGE?
> 
> I've tried removing PGE from XEN_MINIMAL_CR4 but it made no noticeable
> performance difference, so I left it as-is.

My concern isn't about performance, but correctness. I admit I
forgot for a moment that we now always write CR4 (unless the
cached value matches the intended new one). Yet
mmu_cr4_features (starting out as XEN_MINIMAL_CR4) is still of
concern.

I think this at least requires extending the description to
discuss the correctness.

Jan

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