Xen devel - Jul 2006 - RE: Why gnttab_grant_foreign_access() can only grantaccess for one page?

> 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
regions.

Ian

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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?

thanks,
lily

On 7/7/06, Ian Pratt <m+Ian.Pratt@cl.cam.ac.uk>
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
> regions.
>
> Ian
>

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