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Re: [Xen-devel] Why gnttab_grant_foreign_access() can only grantaccess for one page?

  • To: "Ian Pratt" <m+Ian.Pratt@xxxxxxxxxxxx>
  • From: "Lily Huang" <ushuanglily@xxxxxxxxx>
  • Date: Fri, 7 Jul 2006 12:40:20 -0400
  • Cc: Randy Thelen <rthelen@xxxxxxxxxx>, xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>
  • Delivery-date: Fri, 07 Jul 2006 09:40:44 -0700
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  • List-id: Xen developer discussion <xen-devel.lists.xensource.com>

In my case, I often need to allow client domain to remap 16 pages. I don't have exact number of the frequency, but it should be pretty high because I need to allow
two domains to share bulk of data frequently.

I just read the source code. gnttab_grant_foreign_access() seems to just set some data in an in-memory structure "shared".  No hypercall is needed here. Right?
If so, whether calling this function in a batch mode does not make big difference to me. I thought a
gnttab_grant_foreign_access() will cause a hypercall and context switch, then
whether we can do batch granting will be important to performance.

Ian, you said "
grant tables are not typically used to map machine contiguous regions". Does Xen provide any mechanism to map contiguous regions?

Another question: Grant table currently seems to only allow 1024 entries, which means a domain may grant remote access on at most 1024 pages. However, this may not be enough in some cases. For example, in my case, I
need to allow a backend VM to share many pages with frontend. I can certainly change the xen code to make the grant table larger. But does Xen has a better way to handle this situation?


On 7/7/06, Ian Pratt <m+Ian.Pratt@xxxxxxxxxxxx> wrote:
> Lily Huang wrote:
> > THis seems to be less efficient. Why cannot we allow this function
> > to grant access for mulitiple pages just remap function?
> With what frequency are multiple page grants requested?

You can always use multicalls for batches. In the current usage
scenarios, grant tables are not typically used to map machine contiguous


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