Re: [Xen-devel] [PATCH for-next 07/16] xen/arm: Introduce copy_to_guest_phys_flush_dcache

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Fri, 8 Dec 2017, Julien Grall wrote:
> On 06/12/17 12:27, Julien Grall wrote:
> > On 12/06/2017 01:26 AM, Stefano Stabellini wrote:
> > > On Thu, 23 Nov 2017, Julien Grall wrote:
> > > > Hi Andrew,
> > > > 
> > > > On 23/11/17 18:49, Andrew Cooper wrote:
> > > > > On 23/11/17 18:32, Julien Grall wrote:
> > > > > > This new function will be used in a follow-up patch to copy data to
> > > > > > the
> > > > > > guest
> > > > > > using the IPA (aka guest physical address) and then clean the cache.
> > > > > > 
> > > > > > Signed-off-by: Julien Grall <julien.grall@xxxxxxxxxx>
> > > > > > ---
> > > > > >    xen/arch/arm/guestcopy.c           | 10 ++++++++++
> > > > > >    xen/include/asm-arm/guest_access.h |  6 ++++++
> > > > > >    2 files changed, 16 insertions(+)
> > > > > > 
> > > > > > diff --git a/xen/arch/arm/guestcopy.c b/xen/arch/arm/guestcopy.c
> > > > > > index be53bee559..7958663970 100644
> > > > > > --- a/xen/arch/arm/guestcopy.c
> > > > > > +++ b/xen/arch/arm/guestcopy.c
> > > > > > @@ -110,6 +110,16 @@ unsigned long raw_copy_from_guest(void *to,
> > > > > > const
> > > > > > void __user *from, unsigned le
> > > > > >                          COPY_from_guest | COPY_linear);
> > > > > >    }
> > > > > >    +unsigned long copy_to_guest_phys_flush_dcache(struct domain *d,
> > > > > > +                                              paddr_t gpa,
> > > > > > +                                              void *buf,
> > > > > > +                                              unsigned int len)
> > > > > > +{
> > > > > > +    /* P2M is shared between all vCPUs, so the vCPU used does not
> > > > > > matter.
> > > > > > */
> > > > > 
> > > > > Be very careful with this line of thinking.  It is only works after
> > > > > DOMCTL_max_vcpus has succeeded, and before that point, it is a latent
> > > > > NULL pointer dereference.
> > > > 
> > > > I really don't expect that function been used before DOMCT_max_vcpus is
> > > > set.
> > > > It is only used for hardware emulation or Xen loading image into the
> > > > hardware
> > > > domain memory. I could add a check d->vcpus to be safe.
> > > > 
> > > > > 
> > > > > Also, what about vcpus configured with alternative views?
> > > > 
> > > > It is not important because the underlying call is get_page_from_gfn
> > > > that does
> > > > not care about the alternative view (that function take a domain in
> > > > parameter). I can update the comment.
> > > Since this is a new function, would it make sense to take a struct
> > > vcpu* as parameter, instead of a struct domain* ?
> > 
> > Well, I suggested this patch this way because likely everyone will use with
> > d->vcpus[0]. And then you would have to wonder why d->vcpus[0] and not
> > d->vcpus[1]...
> 
> Thinking a bit more to this, it might be better/safer to pass either a domain
> or a vCPU to copy_guest. I can see 2 solutions:
>       1# Introduce a union that use the same parameter:
>               union
>               {
>                       struct
>                       {
>                               struct domain *d;
>                       } ipa;
>                       struct
>                       {
>                               struct vcpu *v;
>                       } gva;
>               }
>         The structure here would be to ensure that it is clear that only
> domain (resp. vcpu) should be used with ipa (resp. gva).
> 
>       2# Have 2 parameters, vcpu and domain.
> 
> Any opinions?

I think that would be clearer. You could also add a paddr_t/vaddr_t
correspondingly inside the union maybe.

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