Re: [Xen-devel] [PATCH v2 1/6] x86/xpti: avoid copying L4 page table contents when possible

[Juergen Gross <jgross@xxxxxxxx>]

On 06/03/18 08:58, Jan Beulich wrote:
>>>> On 06.03.18 at 08:01, <jgross@xxxxxxxx> wrote:
>> On 05/03/18 17:43, Jan Beulich wrote:
>>>>>> On 02.03.18 at 09:13, <jgross@xxxxxxxx> wrote:
>>>> --- a/xen/arch/x86/mm.c
>>>> +++ b/xen/arch/x86/mm.c
>>>> @@ -509,6 +509,8 @@ void make_cr3(struct vcpu *v, mfn_t mfn)
>>>>  
>>>>  void write_ptbase(struct vcpu *v)
>>>>  {
>>>> +    get_cpu_info()->root_pgt_changed = this_cpu(root_pgt) && 
>>>> is_pv_vcpu(v) &&
>>>> +                                       !is_pv_32bit_vcpu(v);
>>>
>>> Why is_pv_vcpu() when you already check is_pv_32bit_vcpu()?
>>
>> I check !is_pv_32bit_vcpu() to catch 64-bit pv-domains only.
> 
> Oh, I'm sorry - For whatever reason I've ignored the ! there.
> 
>>> And don't you need to disallow updating L4s of running guests now
>>> (which is a bad idea anyway)?
>>
>> Yes, I should do that.
> 
> But please do this as a separate change, as strictly speaking this is
> a behavioral change that we can't allow. But I think we simply need
> to accept this (and perhaps uniformly for all page table levels).

Hmm, thinking more about it: wouldn't it be enough to do the change and
then sending a FLUSH_ROOT_PGTBL to the vcpu(s) the affected guest is
running on? This would cause a cycle through the hypervisor resulting in
an updated root page table of that guest.


Juergen

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