Re: [Xen-devel] [PATCH] x86/pagewalk: Remove opt_allow_superpage check from guest_can_use_l2_superpages()

[Tim Deegan <tim@xxxxxxx>]

At 16:00 +0100 on 25 Jul (1500998413), Andrew Cooper wrote:
> The purpose of guest_walk_tables() is to match the behaviour of real hardware.
> 
> A PV guest can have 2M superpages in its pagetables, via the M2P and the
> initial initrd mapping, even if it isn't permitted to create arbitrary 2M
> superpage mappings.

Can the domain builder (or Xen?) really give a guest superpage
mappings for its initrd?  Wouldn't that cause problems for live
migration?

In any case this patch looks correct: the presence of superpages in PV
pagetables is decided by the PV MM rules, so the walker should accept them.

Reviewed-by: Tim Deegan <tim@xxxxxxx>

> guest_can_use_l2_superpages() checking opt_allow_superpage is a piece of PV
> guest policy enforcement, rather than its intended purpose of meaning "would
> hardware tolerate finding an L2 superpage with these control settings?"
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> ---
> CC: Jan Beulich <JBeulich@xxxxxxxx>
> CC: Tim Deegan <tim@xxxxxxx>
> CC: George Dunlap <george.dunlap@xxxxxxxxxxxxx>
> CC: Wei Liu <wei.liu2@xxxxxxxxxx>
> ---
>  xen/include/asm-x86/guest_pt.h | 12 +++++++-----
>  1 file changed, 7 insertions(+), 5 deletions(-)
> 
> diff --git a/xen/include/asm-x86/guest_pt.h b/xen/include/asm-x86/guest_pt.h
> index 72126d5..08031c8 100644
> --- a/xen/include/asm-x86/guest_pt.h
> +++ b/xen/include/asm-x86/guest_pt.h
> @@ -205,15 +205,17 @@ static inline guest_l4e_t guest_l4e_from_gfn(gfn_t gfn, 
> u32 flags)
>  static inline bool guest_can_use_l2_superpages(const struct vcpu *v)
>  {
>      /*
> +     * PV guests use Xen's paging settings.  Being 4-level, 2M
> +     * superpages are unconditionally supported.
> +     *
>       * The L2 _PAGE_PSE bit must be honoured in HVM guests, whenever
>       * CR4.PSE is set or the guest is in PAE or long mode.
>       * It's also used in the dummy PT for vcpus with CR0.PG cleared.
>       */
> -    return (is_pv_vcpu(v)
> -            ? opt_allow_superpage
> -            : (GUEST_PAGING_LEVELS != 2
> -               || !hvm_paging_enabled(v)
> -               || (v->arch.hvm_vcpu.guest_cr[4] & X86_CR4_PSE)));
> +    return (is_pv_vcpu(v) ||
> +            GUEST_PAGING_LEVELS != 2 ||
> +            !hvm_paging_enabled(v) ||
> +            (v->arch.hvm_vcpu.guest_cr[4] & X86_CR4_PSE));
>  }
>  
>  static inline bool guest_can_use_l3_superpages(const struct domain *d)
> -- 
> 2.1.4
> 

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