# Re: \[PATCH V4 08/13\] hyperv/vmbus: Initialize VMbus ring buffer for Isolation VM

- From: Tianyu Lan \<hidden\>
- Date: 2021-09-02 13:36:11 
- List: netdev
- Page: https://ratatoskr.run/netdev/2021/09/1495800
- Thread: https://ratatoskr.run/xen-devel/2021/08/1490756/t.md

```text
On 9/2/2021 8:23 AM, Michael Kelley wrote:
>> +	} else {
>> +		pages_wraparound = kcalloc(page_cnt * 2 - 1,
>> +					   sizeof(struct page *),
>> +					   GFP_KERNEL);
>> +
>> +		pages_wraparound[0] = pages;
>> +		for (i = 0; i < 2 * (page_cnt - 1); i++)
>> +			pages_wraparound[i + 1] =
>> +				&pages[i % (page_cnt - 1) + 1];
>> +
>> +		ring_info->ring_buffer = (struct hv_ring_buffer *)
>> +			vmap(pages_wraparound, page_cnt * 2 - 1, VM_MAP,
>> +				PAGE_KERNEL);
>> +
>> +		kfree(pages_wraparound);
>> +		if (!ring_info->ring_buffer)
>> +			return -ENOMEM;
>> +	}
> With this patch, the code is a big "if" statement with two halves -- one
> when SNP isolation is in effect, and the other when not.  The SNP isolation
> case does the work using PFNs with the shared_gpa_boundary added,
> while the other case does the same work but using struct page.  Perhaps
> I'm missing something, but can both halves be combined and always
> do the work using PFNs?  The only difference is whether to add the
> shared_gpa_boundary, and whether to zero the memory when done.
> So get the starting PFN, then have an "if" statement for whether to
> add the shared_gpa_boundary.  Then everything else is the same.
> At the end, use an "if" statement to decide whether to zero the
> memory.  It would really be better to have the logic in this algorithm
> coded only once.
>

Hi Michael:
	I have tried this before. But vmap_pfn() only works for those pfns out of normal memory. Please see vmap_pfn_apply() for detail and
return error when the PFN is valid.


```