Re: [Xen-devel] [PATCH v4 02/12] x86/mm: add HYPERVISOR_memory_op to acquire guest resources

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Thu, Sep 07, 2017 at 12:18:25PM +0100, Paul Durrant wrote:
> > -----Original Message-----
> > From: Wei Liu [mailto:wei.liu2@xxxxxxxxxx]
> > Sent: 07 September 2017 12:11
> > To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>
> > Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx; Andrew Cooper
> > <Andrew.Cooper3@xxxxxxxxxx>; Jan Beulich <jbeulich@xxxxxxxx>; Wei Liu
> > <wei.liu2@xxxxxxxxxx>
> > Subject: Re: [Xen-devel] [PATCH v4 02/12] x86/mm: add
> > HYPERVISOR_memory_op to acquire guest resources
> > 
> > On Tue, Sep 05, 2017 at 12:37:06PM +0100, Paul Durrant wrote:
> > [...]
> > >
> > > +static int xenmem_acquire_grant_table(struct domain *d,
> > > +                                      unsigned long frame,
> > > +                                      unsigned long nr_frames,
> > > +                                      unsigned long mfn_list[])
> > > +{
> > > +    unsigned int i;
> > > +
> > > +    /*
> > > +     * Iterate through the list backwards so that gnttab_get_frame() is
> > > +     * first called for the highest numbered frame. This means that the
> > > +     * out-of-bounds check will be done on the first iteration and, if
> > > +     * the table needs to grow, it will only grow once.
> > > +     */
> > > +    i = nr_frames;
> > > +    while ( i-- != 0 )
> > > +    {
> > > +        mfn_t mfn = gnttab_get_frame(d, frame + i);
> > > +
> > 
> > I think you should lock guest grant table first and use the _locked
> > variant here to get a consistent view of guest grant table frames.
> 
> Once the table has grown, is there any way they can change?
> 

Hmm... no.

I think you can leave the code as-is.

> 
> Ok, if you think it's necessary. (This is a tools-only hypercall and the 
> ranges are supplied by privcmd, allocated in kernel).
> 

IMHO we should allow for use case for semi-trusted users of this
hypercall in the future.

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