Re: [Xen-devel] [PATCH 4/4] vm_event: Add support for multi-page ring buffer

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 13/09/18 16:02, Petre Pircalabu wrote:
> In high throughput introspection scenarios where lots of monitor
> vm_events are generated, the ring buffer can fill up before the monitor
> application gets a chance to handle all the requests thus blocking
> other vcpus which will have to wait for a slot to become available.
>
> This patch adds support for extending the ring buffer by allocating a
> number of pages from domheap and mapping them to the monitor
> application's domain using the foreignmemory_map_resource interface.
> Unlike the current implementation, the ring buffer pages are not part of
> the introspected DomU, so they will not be reclaimed when the monitor is
> disabled.
>
> Signed-off-by: Petre Pircalabu <ppircalabu@xxxxxxxxxxxxxxx>

What about the slotted format for the synchronous events?  While this is
fine for the async bits, I don't think we want to end up changing the
mapping API twice.

Simply increasing the size of the ring puts more pressure on the

> diff --git a/xen/arch/x86/domain_page.c b/xen/arch/x86/domain_page.c
> index 0d23e52..2a9cbf3 100644
> --- a/xen/arch/x86/domain_page.c
> +++ b/xen/arch/x86/domain_page.c
> @@ -331,10 +331,9 @@ void *__map_domain_pages_global(const struct page_info 
> *pg, unsigned int nr)
>  {
>      mfn_t mfn[nr];
>      int i;
> -    struct page_info *cur_pg = (struct page_info *)&pg[0];
>  
>      for (i = 0; i < nr; i++)
> -        mfn[i] = page_to_mfn(cur_pg++);
> +        mfn[i] = page_to_mfn(pg++);

This hunk looks like it should be in the previous patch?  That said...

>  
>      return map_domain_pages_global(mfn, nr);
>  }
> diff --git a/xen/common/vm_event.c b/xen/common/vm_event.c
> index 4793aac..faece3c 100644
> --- a/xen/common/vm_event.c
> +++ b/xen/common/vm_event.c
> @@ -39,16 +39,66 @@
>  #define vm_event_ring_lock(_ved)       spin_lock(&(_ved)->ring_lock)
>  #define vm_event_ring_unlock(_ved)     spin_unlock(&(_ved)->ring_lock)
>  
> +#define XEN_VM_EVENT_ALLOC_FROM_DOMHEAP 0xFFFFFFFF
> +
> +static int vm_event_alloc_ring(struct domain *d, struct vm_event_domain *ved)
> +{
> +    struct page_info *page;
> +    void *va = NULL;
> +    int i, rc = -ENOMEM;
> +
> +    page = alloc_domheap_pages(d, ved->ring_order, MEMF_no_refcount);
> +    if ( !page )
> +        return -ENOMEM;

... what is wrong with vzalloc()?

You don't want to be making a ring_order allocation, especially as the
order grows.  All you need are some mappings which are virtually
contiguous, not physically contiguous.

~Andrew

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