Re: [Xen-devel] [PATCH 8/8] xen/x86: Additional SMAP modes to work around buggy 32bit PV guests

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 24.06.15 at 18:31, <andrew.cooper3@xxxxxxxxxx> wrote:
> @@ -784,6 +791,11 @@ unsigned long pv_guest_cr4_to_real_cr4(const struct vcpu 
> *v)
>      if ( v->domain->arch.vtsc )
>          cr4 |= X86_CR4_TSD;
>  
> +    /* Disable SMAP behind unaware 32bit PV guests. */
> +    if ( (smap_mode == smap_mode_compat) && is_pv_32bit_vcpu(v) &&
> +         ((v->arch.pv_vcpu.ctrlreg[4] & X86_CR4_SMAP) == 0) )
> +        cr4 &= ~X86_CR4_SMAP;

There's actually another problem here: The function you modify
is used by paravirt_ctxt_switch_to(). Obviously you will want to
make sure to re-enable SMAP when switching away from the
guest, and not only at the point you finally switch to a suitable
other non-idle vCPU.

And then there's the question whether the above fixup wouldn't
better be deferred until compat_restore_all_guest, minimizing
the "damage" to Xen?

Jan


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