Re: [PATCH v6 01/12] IOMMU/x86: support freeing of pagetables

[Jan Beulich <jbeulich@xxxxxxxx>]

On 28.06.2022 14:39, Roger Pau Monné wrote:
> On Thu, Jun 09, 2022 at 12:16:38PM +0200, Jan Beulich wrote:
>> For vendor specific code to support superpages we need to be able to
>> deal with a superpage mapping replacing an intermediate page table (or
>> hierarchy thereof). Consequently an iommu_alloc_pgtable() counterpart is
>> needed to free individual page tables while a domain is still alive.
>> Since the freeing needs to be deferred until after a suitable IOTLB
>> flush was performed, released page tables get queued for processing by a
>> tasklet.
>>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>> ---
>> I was considering whether to use a softirq-tasklet instead. This would
>> have the benefit of avoiding extra scheduling operations, but come with
>> the risk of the freeing happening prematurely because of a
>> process_pending_softirqs() somewhere.
>> ---
>> v6: Extend comment on the use of process_pending_softirqs().
>> v5: Fix CPU_UP_PREPARE for BIGMEM. Schedule tasklet in CPU_DOWN_FAILED
>>     when list is not empty. Skip all processing in CPU_DEAD when list is
>>     empty.
>> v4: Change type of iommu_queue_free_pgtable()'s 1st parameter. Re-base.
>> v3: Call process_pending_softirqs() from free_queued_pgtables().
>>
>> --- a/xen/arch/x86/include/asm/iommu.h
>> +++ b/xen/arch/x86/include/asm/iommu.h
>> @@ -147,6 +147,7 @@ void iommu_free_domid(domid_t domid, uns
>>  int __must_check iommu_free_pgtables(struct domain *d);
>>  struct domain_iommu;
>>  struct page_info *__must_check iommu_alloc_pgtable(struct domain_iommu *hd);
>> +void iommu_queue_free_pgtable(struct domain_iommu *hd, struct page_info 
>> *pg);
>>  
>>  #endif /* !__ARCH_X86_IOMMU_H__ */
>>  /*
>> --- a/xen/drivers/passthrough/x86/iommu.c
>> +++ b/xen/drivers/passthrough/x86/iommu.c
>> @@ -12,6 +12,7 @@
>>   * this program; If not, see <http://www.gnu.org/licenses/>.
>>   */
>>  
>> +#include <xen/cpu.h>
>>  #include <xen/sched.h>
>>  #include <xen/iocap.h>
>>  #include <xen/iommu.h>
>> @@ -551,6 +552,103 @@ struct page_info *iommu_alloc_pgtable(st
>>      return pg;
>>  }
>>  
>> +/*
>> + * Intermediate page tables which get replaced by large pages may only be
>> + * freed after a suitable IOTLB flush. Hence such pages get queued on a
>> + * per-CPU list, with a per-CPU tasklet processing the list on the 
>> assumption
>> + * that the necessary IOTLB flush will have occurred by the time tasklets 
>> get
>> + * to run. (List and tasklet being per-CPU has the benefit of accesses not
>> + * requiring any locking.)
>> + */
>> +static DEFINE_PER_CPU(struct page_list_head, free_pgt_list);
>> +static DEFINE_PER_CPU(struct tasklet, free_pgt_tasklet);
>> +
>> +static void free_queued_pgtables(void *arg)
> 
> I think this is missing a cf_check attribute?

Oh, indeed - thanks for spotting. We're still lacking compiler detection
of such issues.

>> +{
>> +    struct page_list_head *list = arg;
>> +    struct page_info *pg;
>> +    unsigned int done = 0;
> 
> Might be worth adding an:
> 
> ASSERT(list == &this_cpu(free_pgt_list));
> 
> To make sure tasklets are never executed on the wrong CPU.

Sure, added.

> Apart form that:
> 
> Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Thanks.

Jan