Re: [Xen-devel] [PATCH 17/24] xen: allow privcmd for HVM guests

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Fri, Jul 27, 2012 at 03:10:13PM +0100, Stefano Stabellini wrote:
> On Fri, 27 Jul 2012, Jan Beulich wrote:
> > >>> On 26.07.12 at 17:33, Stefano Stabellini 
> > >>> <stefano.stabellini@xxxxxxxxxxxxx> wrote:
> > > In order for privcmd mmap to work correctly, xen_remap_domain_mfn_range
> > > needs to be implemented for HVM guests.
> > > If it is not, mmap is going to fail later on.
> > 
> > Somehow, for me at least, this description doesn't connect to the
> > actual change.
> 
> We can remove the "return -ENOSYS" from privcmd_mmap but the actual mmap
> is still not going to work unless xen_remap_domain_mfn_range is
> implemented correctly.
> The x86 implementation of xen_remap_domain_mfn_range is PV only so it is
> not going to work for HVM or auto_translated_physmap guests.
> As a result mmap_batch_fn is going to fail.

So what you are saying is that this check is redundant and that earlier
on in the call stack this check is made?

I am not seeing it? I am seeing an:

289         if (!xen_initial_domain())
290                 return -EPERM;

But that would still work.

Perhaps adding an:

        if (xen_hvm_domain())
                return -ENOSYS

is more appropiate in privcmd_ioctl_mmap_batch?

Irrespective of HVM guests, I recall that it is possible to run PV guests
with XENFEAT_auto_translated_physmap? How will this be impacted?

> 
> 
> > > Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
> > > ---
> > >  drivers/xen/privcmd.c |    4 ----
> > >  1 files changed, 0 insertions(+), 4 deletions(-)
> > > 
> > > diff --git a/drivers/xen/privcmd.c b/drivers/xen/privcmd.c
> > > index ccee0f1..85226cb 100644
> > > --- a/drivers/xen/privcmd.c
> > > +++ b/drivers/xen/privcmd.c
> > > @@ -380,10 +380,6 @@ static struct vm_operations_struct privcmd_vm_ops = {
> > >  
> > >  static int privcmd_mmap(struct file *file, struct vm_area_struct *vma)
> > >  {
> > > - /* Unsupported for auto-translate guests. */
> > > - if (xen_feature(XENFEAT_auto_translated_physmap))
> > > -         return -ENOSYS;
> > > -
> > 
> > Is this safe on x86?
> > 
> 
> It is safe in the sense that is not going to crash dom0 or the
> hypervisor, but it is not going to work.
> 
> Actually in order for it to be safe we need this additional change:
> 
> diff --git a/arch/x86/xen/mmu.c b/arch/x86/xen/mmu.c
> index 3a73785..885a223 100644
> --- a/arch/x86/xen/mmu.c
> +++ b/arch/x86/xen/mmu.c
> @@ -2310,6 +2310,9 @@ int xen_remap_domain_mfn_range(struct vm_area_struct 
> *vma,
>       unsigned long range;
>       int err = 0;
>  
> +     if (xen_feature(XENFEAT_auto_translated_physmap))
> +             return -EINVAL;
> +
>       prot = __pgprot(pgprot_val(prot) | _PAGE_IOMAP);
>  
>       BUG_ON(!((vma->vm_flags & (VM_PFNMAP | VM_RESERVED | VM_IO)) ==
> 
> 
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