Re: [Xen-devel] [PATCH v3 5/7] xen/x86: disable global pages for domains with XPTI active

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 21.03.18 at 13:51, <jgross@xxxxxxxx> wrote:
> Instead of flushing the TLB from global pages when switching address
> spaces with XPTI being active just disable global pages via %cr4
> completely when a domain subject to XPTI is active. This avoids the
> need for extra TLB flushes as loading %cr3 will remove all TLB
> entries.

I continue to be not entirely convinced of this move. I had an
alternative in mind: Since retaining global pages is particularly
relevant for switches between guest user and guest kernel
modes, what if we made a shortcut from e.g. lstar_enter through
switch_to_kernel to restore_all_guest without ever switching to
the full page Xen tables?

> --- a/xen/arch/x86/mm.c
> +++ b/xen/arch/x86/mm.c
> @@ -508,18 +508,23 @@ void make_cr3(struct vcpu *v, mfn_t mfn)
>  void write_ptbase(struct vcpu *v)
>  {
>      struct cpu_info *cpu_info = get_cpu_info();
> +    unsigned long new_cr4;
> +
> +    new_cr4 = (is_pv_vcpu(v) && !is_idle_vcpu(v))
> +              ? pv_guest_cr4_to_real_cr4(v) : mmu_cr4_features;

I'm not overly happy to see any new uses of mmu_cr4_features.
This should really only be used for priming certain values imo,
which isn't the case here (otoh pv_guest_cr4_to_real_cr4() does
so too, and perhaps better wouldn't). Hence I wonder whether
this shouldn't be read_cr4() | X86_CR4_PGE, not the least
because we've just got rid of the blanket reversion to
mmu_cr4_features in VMX code.

Jan


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