Re: [Xen-devel] [PATCH v2] x86: do not enable global pages when virtualized on AMD hardware

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Wed, Dec 04, 2019 at 02:15:58PM +0100, Jan Beulich wrote:
> On 04.12.2019 12:52, Roger Pau Monné wrote:
> > On Wed, Dec 04, 2019 at 12:05:35PM +0100, Jan Beulich wrote:
> >> On 04.12.2019 11:44, Roger Pau Monne wrote:
> >>> --- a/xen/arch/x86/pv/domain.c
> >>> +++ b/xen/arch/x86/pv/domain.c
> >>> @@ -118,11 +118,18 @@ unsigned long pv_fixup_guest_cr4(const struct vcpu 
> >>> *v, unsigned long cr4)
> >>>              (mmu_cr4_features & PV_CR4_GUEST_VISIBLE_MASK));
> >>>  }
> >>>  
> >>> +static int opt_global_pages = -1;
> >>
> >> int8_t __read_mostly
> >>
> >>> +boolean_runtime_param("global-pages", opt_global_pages);
> >>> +
> >>>  unsigned long pv_make_cr4(const struct vcpu *v)
> >>>  {
> >>>      const struct domain *d = v->domain;
> >>>      unsigned long cr4 = mmu_cr4_features &
> >>>          ~(X86_CR4_PCIDE | X86_CR4_PGE | X86_CR4_TSD);
> >>> +    bool pge = opt_global_pages == -1 ? (!cpu_has_hypervisor ||
> >>> +                                         boot_cpu_data.x86_vendor !=
> >>> +                                         X86_VENDOR_AMD)
> >>> +                                      : !!opt_global_pages;
> >>
> >> Let's avoid re-doing this evaluation each time we come here.
> >> Post boot the value can only change to 0 or 1. Hence in some
> >> __init function you can apply the default calculation done
> >> here.
> > 
> > I've assumed that boolean_runtime_param can be changed during runtime
> > by the user, and hence the value calculated at boot time would become
> > stale if the user changes it after boot, which should be fine for this
> > option.
> 
> I'm afraid I can't decide whether you agree or disagree with my
> comment.

Oh sorry fro not being clear. I was meant to disagree, calculating a
value at init time would be wrong, since opt_global_pages can change
during runtime.

Thanks, Roger.

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