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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 09.12.2019 11:20, Roger Pau Monné wrote:
> On Wed, Dec 04, 2019 at 06:05:11PM +0100, Jan Beulich wrote:
>> 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.
> 
> Would you be fine with adding the following at the end of the commit
> message.
> 
> "Note that XEN_MINIMAL_CR4 is not modified, and thus global pages are
> left enabled for the hypervisor. This is not an issue because the code
> to switch the control registers (cr3 and cr4) already takes into
> account such situation and performs the necessary flushes. The same
> already happens when using XPTI or PCIDE, as the guest cr4 doesn't
> have global pages enabled in that case either."

Yes, this is good for XEN_MINIMAL_CR4. But I think mmu_cr4_features
needs discussing (or at least mentioning, if the same arguments
apply) as well.

Jan

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