Re: [Xen-devel] [PATCH v3 1/7] x86/xpti: avoid copying L4 page table contents when possible

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 22.03.18 at 16:26, <jgross@xxxxxxxx> wrote:
> On 22/03/18 15:31, Jan Beulich wrote:
>>>>> On 21.03.18 at 13:51, <jgross@xxxxxxxx> wrote:
>>>  void write_ptbase(struct vcpu *v)
>>>  {
>>> +    if ( this_cpu(root_pgt) && is_pv_vcpu(v) && !is_pv_32bit_vcpu(v) )
>>> +        get_cpu_info()->root_pgt_changed = true;
>>>      write_cr3(v->arch.cr3);
>> 
>> When you come here from e.g. __sync_local_execstate(), you
>> don't really need to set the flag. Of course you'll come here again
>> before the next 64-bit PV vCPU will make it to restore_all_guest,
>> so by the time we make it there the flag will be set anyway.
>> However, if you already use such a subtlety, then there's also
>> no point excluding 32-bit vCPU-s here (nor in make_cr3()), as
>> those will never make it to restore_all_guest. Same then for
>> excluding HVM vCPU-s. And I then wonder whether (here or
>> more likely in a later patch) the root_pgt check couldn't go away
>> as well.
> 
> I'm not sure this is worth it. Patch 3 will re-introduce a conditional
> here and it will look rather different (e.g. without the root_pgt
> check). So micro-optimizing this patch barely makes any sense.

Yes, I've seen that once I made it there. Perhaps worth dropping
the two is_*() checks here, but not worry about the root_pgt one.

Jan


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