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

[Juergen Gross <jgross@xxxxxxxx>]

On 27/03/18 09:23, Jan Beulich wrote:
>>>> On 27.03.18 at 09:14, <jgross@xxxxxxxx> wrote:
>> On 22/03/18 17:30, Jan Beulich wrote:
>>>>>> 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.
>>
>> I just realized that using read_cr4() | X86_CR4_PGE is wrong. We would
>> keep bits switched on which a pv domain is allowed to modify (plus
>> CR4_TSD eventually).
>>
>> Do we really want that?
> 
> Does it matter what exact CR4 settings we run with when it's not
> a PV guest that's in context, and when we don't depend on the
> settings ourselves? I don't think it does, and HVM guests run with
> their own CR4 anyway. In fact there may end up being cases
> where we won't need to switch CR4 another time when we come
> here the next time with v being a PV vCPU.

I could imagine that there is some performance impact. cr4.tsd set
might make rdtsc a little bit slower as an additional privilege level
check is needed.

Suspending requires cr4.fsgsbase to be set, which might have been
reset by a pv guest.


Juergen


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel