Re: [PATCH v4 07/21] IOMMU/x86: support freeing of pagetables

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Mon, Apr 25, 2022 at 10:35:45AM +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.

I'm sorry again if I already raised this, I don't seem to find a
reference.

What about doing the freeing before resuming the guest execution in
guest vCPU context?

We already have a hook like this on HVM in hvm_do_resume() calling
vpci_process_pending().  I wonder whether we could have a similar hook
for PV and keep the pages to be freed in the vCPU instead of the pCPU.
This would have the benefit of being able to context switch the vCPU
in case the operation takes too long.

Not that the current approach is wrong, but doing it in the guest
resume path we could likely prevent guests doing heavy p2m
modifications from hogging CPU time.

> ---
> 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/iommu.h>
>  #include <xen/paging.h>
> @@ -550,6 +551,91 @@ 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)
> +{
> +    struct page_list_head *list = arg;
> +    struct page_info *pg;
> +    unsigned int done = 0;
> +

With the current logic I think it might be helpful to assert that the
list is not empty when we get here?

Given the operation requires a context switch we would like to avoid
such unless there's indeed pending work to do.

> +    while ( (pg = page_list_remove_head(list)) )
> +    {
> +        free_domheap_page(pg);
> +
> +        /* Granularity of checking somewhat arbitrary. */
> +        if ( !(++done & 0x1ff) )
> +             process_pending_softirqs();
> +    }
> +}
> +
> +void iommu_queue_free_pgtable(struct domain_iommu *hd, struct page_info *pg)
> +{
> +    unsigned int cpu = smp_processor_id();
> +
> +    spin_lock(&hd->arch.pgtables.lock);
> +    page_list_del(pg, &hd->arch.pgtables.list);
> +    spin_unlock(&hd->arch.pgtables.lock);
> +
> +    page_list_add_tail(pg, &per_cpu(free_pgt_list, cpu));
> +
> +    tasklet_schedule(&per_cpu(free_pgt_tasklet, cpu));
> +}
> +
> +static int cf_check cpu_callback(
> +    struct notifier_block *nfb, unsigned long action, void *hcpu)
> +{
> +    unsigned int cpu = (unsigned long)hcpu;
> +    struct page_list_head *list = &per_cpu(free_pgt_list, cpu);
> +    struct tasklet *tasklet = &per_cpu(free_pgt_tasklet, cpu);
> +
> +    switch ( action )
> +    {
> +    case CPU_DOWN_PREPARE:
> +        tasklet_kill(tasklet);
> +        break;
> +
> +    case CPU_DEAD:
> +        page_list_splice(list, &this_cpu(free_pgt_list));

I think you could check whether list is empty before queuing it?

Thanks, Roger.