Re: [Xen-devel] [PATCH 2/3] x86/altp2m: Add a hvmop for setting the suppress #VE bit

[Adrian Pop <apop@xxxxxxxxxxxxxxx>]

Hello,

On Mon, May 29, 2017 at 08:38:33AM -0600, Jan Beulich wrote:
> >>> On 18.05.17 at 17:07, <apop@xxxxxxxxxxxxxxx> wrote:
> > --- a/xen/arch/x86/mm/mem_access.c
> > +++ b/xen/arch/x86/mm/mem_access.c
> > @@ -466,6 +466,54 @@ int p2m_get_mem_access(struct domain *d, gfn_t gfn, 
> > xenmem_access_t *access)
> >  }
> >  
> >  /*
> > + * Set/clear the #VE suppress bit for a page.  Only available on VMX.
> > + */
> > +int p2m_set_suppress_ve(struct domain *d, gfn_t gfn, uint8_t suppress_ve,
> 
> suppress_ve presumably is meant to be boolean.

Yes.  It can be changed to bool.

> > +                        unsigned int altp2m_idx)
> > +{
> > +    struct p2m_domain *host_p2m = p2m_get_hostp2m(d);
> > +    struct p2m_domain *ap2m = NULL;
> > +    struct p2m_domain *p2m = NULL;
> 
> Pointless initializer.

Ok.

> > +    mfn_t mfn;
> > +    p2m_access_t a;
> > +    p2m_type_t t;
> > +    unsigned long gfn_l;
> 
> Please avoid this local variable and use gfn_x() in the two places
> you need to.

Sure.

> > +    int rc = 0;
> 
> Pointless initializer again.
 
Right.

> > +
> > +    if ( !cpu_has_vmx )
> > +        return -EOPNOTSUPP;
> 
> Is this enough? Wouldn't it be better to signal the caller whenever
> hardware (or even software) isn't going to honor the request?

Well, the caller checks the return value.  The libxc function
xc_altp2m_set_suppress_ve for instance will return a negative in this
case.


> > +    if ( altp2m_idx > 0 )
> > +    {
> > +        if ( altp2m_idx >= MAX_ALTP2M ||
> > +                d->arch.altp2m_eptp[altp2m_idx] == mfn_x(INVALID_MFN) )
> 
> Indentation.

Ok.

> > +            return -EINVAL;
> > +
> > +        p2m = ap2m = d->arch.altp2m_p2m[altp2m_idx];
> > +    }
> > +    else
> > +    {
> > +        p2m = host_p2m;
> > +    }
> 
> Unnecessary braces.
 
Ok.

> > +    p2m_lock(host_p2m);
> > +    if ( ap2m )
> > +        p2m_lock(ap2m);
> > +
> > +    gfn_l = gfn_x(gfn);
> > +    mfn = p2m->get_entry(p2m, gfn_l, &t, &a, 0, NULL, NULL);
> > +    if ( !mfn_valid(mfn) )
> > +        return -ESRCH;
> > +    rc = p2m->set_entry(p2m, gfn_l, mfn, PAGE_ORDER_4K, t, a,
> > +                        suppress_ve);
> > +    if ( ap2m )
> > +        p2m_unlock(ap2m);
> > +    p2m_unlock(host_p2m);
> 
> To fiddle with a single gfn, this looks to be very heavy locking.
> While for now gfn_lock() is the same as p2m_lock(), from an
> abstract perspective I'd expect gfn_lock() to suffice here at 
> least in the non-altp2m case.
 
Ok.

> And then there are two general questions: Without a libxc layer
> function, how is one supposed to use this new sub-op? Is it
> really intended to permit a guest to call this for itself?
 
Well, the sub-op could be used from a Linux kernel module if libxc is
not available if struct xen_hvm_altp2m_op and struct
xen_hvm_altp2m_set_suppress_ve are defined.

Our use case, though, involves either Dom0 or a "privileged" DomU
altering the suppress #VE bit for the target guest.

> Jan
> 

Thanks!

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