Re: [PATCH v2 1/5] x86/paging: fold most HAP and shadow final teardown

[Jan Beulich <jbeulich@xxxxxxxx>]

On 16.03.2023 13:24, Roger Pau Monné wrote:
> On Mon, Jan 09, 2023 at 02:39:19PM +0100, Jan Beulich wrote:
>> HAP does a few things beyond what's common, which are left there at
>> least for now. Common operations, however, are moved to
>> paging_final_teardown(), allowing shadow_final_teardown() to go away.
>>
>> While moving (and hence generalizing) the respective SHADOW_PRINTK()
>> drop the logging of total_pages from the 2nd instance - the value is
>> necessarily zero after {hap,shadow}_set_allocation() - and shorten the
>> messages, in part accounting for PAGING_PRINTK() logging __func__
>> already.
>>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>> ---
>> The remaining parts of hap_final_teardown() could be moved as well, at
>> the price of a CONFIG_HVM conditional. I wasn't sure whether that was
>> deemed reasonable.
>> ---
>> v2: Shorten PAGING_PRINTK() messages. Adjust comments while being
>>     moved.
>>
>> --- a/xen/arch/x86/include/asm/shadow.h
>> +++ b/xen/arch/x86/include/asm/shadow.h
>> @@ -78,9 +78,6 @@ int shadow_domctl(struct domain *d,
>>  void shadow_vcpu_teardown(struct vcpu *v);
>>  void shadow_teardown(struct domain *d, bool *preempted);
>>  
>> -/* Call once all of the references to the domain have gone away */
>> -void shadow_final_teardown(struct domain *d);
>> -
>>  void sh_remove_shadows(struct domain *d, mfn_t gmfn, int fast, int all);
>>  
>>  /* Adjust shadows ready for a guest page to change its type. */
>> --- a/xen/arch/x86/mm/hap/hap.c
>> +++ b/xen/arch/x86/mm/hap/hap.c
>> @@ -268,8 +268,8 @@ static void hap_free(struct domain *d, m
>>  
>>      /*
>>       * For dying domains, actually free the memory here. This way less work 
>> is
>> -     * left to hap_final_teardown(), which cannot easily have preemption 
>> checks
>> -     * added.
>> +     * left to paging_final_teardown(), which cannot easily have preemption
>> +     * checks added.
>>       */
>>      if ( unlikely(d->is_dying) )
>>      {
>> @@ -552,18 +552,6 @@ void hap_final_teardown(struct domain *d
>>      for (i = 0; i < MAX_NESTEDP2M; i++) {
>>          p2m_teardown(d->arch.nested_p2m[i], true, NULL);
>>      }
>> -
>> -    if ( d->arch.paging.total_pages != 0 )
>> -        hap_teardown(d, NULL);
>> -
>> -    p2m_teardown(p2m_get_hostp2m(d), true, NULL);
>> -    /* Free any memory that the p2m teardown released */
>> -    paging_lock(d);
>> -    hap_set_allocation(d, 0, NULL);
>> -    ASSERT(d->arch.paging.p2m_pages == 0);
>> -    ASSERT(d->arch.paging.free_pages == 0);
>> -    ASSERT(d->arch.paging.total_pages == 0);
>> -    paging_unlock(d);
>>  }
>>  
>>  void hap_vcpu_teardown(struct vcpu *v)
>> --- a/xen/arch/x86/mm/paging.c
>> +++ b/xen/arch/x86/mm/paging.c
>> @@ -842,10 +842,45 @@ int paging_teardown(struct domain *d)
>>  /* Call once all of the references to the domain have gone away */
>>  void paging_final_teardown(struct domain *d)
>>  {
>> -    if ( hap_enabled(d) )
>> +    bool hap = hap_enabled(d);
>> +
>> +    PAGING_PRINTK("%pd start: total = %u, free = %u, p2m = %u\n",
>> +                  d, d->arch.paging.total_pages,
>> +                  d->arch.paging.free_pages, d->arch.paging.p2m_pages);
>> +
>> +    if ( hap )
>>          hap_final_teardown(d);
>> +
>> +    /*
>> +     * Remove remaining paging memory.  This can be nonzero on certain error
>> +     * paths.
>> +     */
>> +    if ( d->arch.paging.total_pages )
>> +    {
>> +        if ( hap )
>> +            hap_teardown(d, NULL);
>> +        else
>> +            shadow_teardown(d, NULL);
> 
> For a logical PoV, shouldn't hap_teardown() be called before
> hap_final_teardown()?

Yes and no: The meaning of "final" has changed - previously it meant "the
final parts of tearing down" while now it means "the parts of tearing
down which must be done during final cleanup". I can't think of a better
name, so I left "hap_final_teardown" as it was.

> Also hap_final_teardown() already contains a call to hap_teardown() if
> total_pages != 0, so this is just redundant in the HAP case?

Well, like in shadow_final_teardown() there was such a call prior to this
change, but there's none left now.

> Maybe we want to pull that hap_teardown() out of hap_final_teardown()

That's what I'm doing here.

> and re-order the logic so hap_teardown() is called before
> hap_final_teardown()?

I'm not convinced re-ordering would be correct; even if it was I wouldn't
want to change order of operations here. Instead I want to limit the
changes to just the folding of duplicate (with shadow) code.

Jan