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

["Ian Pratt" <m+Ian.Pratt@xxxxxxxxxxxx>]

> 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.

Yes, only one side of the interface requires hypercalls.

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

Not as yet as it hasn't been needed. Are you sure the pages you want to
map are machine (as opposed to just guest physical) contiguous.

> 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?

There should be a hypercall to grow a domain's grant table. It's been on
the todo list for a while. Want to knock something up?

Thanks,
Ian

> thanks,
> lily
> 
> 
> 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
>       regions.
> 
>       Ian
> 
> 


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