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

[Michael Kelley <mikelley@microsoft.com>]

From: Tianyu Lan <ltykernel@gmail.com> Sent: Thursday, September 2, 2021 6:36 AM
> 
> 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.
> 

Indeed.  This ties into the discussion with Christoph about coming up
with generalized helper functions to assist in handling the
shared_gpa_boundary.   Having a single implementation here in
hv_ringbuffer_init() would be a good goal as well.

Michael