Re: [PATCH v5 06/10] x86/mem-sharing: copy GADDR based shared guest areas

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Wed, Oct 04, 2023 at 12:01:21PM +0100, Julien Grall wrote:
> Hi Roger,
> 
> On 04/10/2023 09:20, Roger Pau Monné wrote:
> > On Tue, Oct 03, 2023 at 04:25:58PM -0400, Tamas K Lengyel wrote:
> > > On Tue, Oct 3, 2023 at 11:07 AM Julien Grall <julien@xxxxxxx> wrote:
> > > > 
> > > > Hi Roger,
> > > > 
> > > > On 03/10/2023 15:29, Roger Pau Monné wrote:
> > > > > On Tue, Oct 03, 2023 at 09:53:11AM -0400, Tamas K Lengyel wrote:
> > > > 
> > > > Tamas, somehow your e-mails don't show up in my inbox (even if I am
> > > > CCed) or even on lore.kernel.org/xen-devel. It is not even in my SPAM
> > > > folder.
> > > 
> > > Thanks, I've switched mailservers, hopefully that resolves the issue.
> 
> It did. Thanks!
> 
> > > 
> > > > 
> > > > > > On Mon, Oct 2, 2023 at 11:13 AM Roger Pau Monne 
> > > > > > <roger.pau@xxxxxxxxxx> wrote:
> > > > > > > 
> > > > > > > From: Jan Beulich <jbeulich@xxxxxxxx>
> > > > > > > 
> > > > > > > In preparation of the introduction of new vCPU operations 
> > > > > > > allowing to
> > > > > > > register the respective areas (one of the two is x86-specific) by
> > > > > > > guest-physical address, add the necessary fork handling (with the
> > > > > > > backing function yet to be filled in).
> > > > > > > 
> > > > > > > Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> > > > > > > Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> > > > > > > ---
> > > > > > > Changes since v4:
> > > > > > >    - Rely on map_guest_area() to populate the child p2m if 
> > > > > > > necessary.
> > > > > > > ---
> > > > > > >    xen/arch/x86/mm/mem_sharing.c | 31 
> > > > > > > +++++++++++++++++++++++++++++++
> > > > > > >    xen/common/domain.c           |  7 +++++++
> > > > > > >    2 files changed, 38 insertions(+)
> > > > > > > 
> > > > > > > diff --git a/xen/arch/x86/mm/mem_sharing.c 
> > > > > > > b/xen/arch/x86/mm/mem_sharing.c
> > > > > > > index 5f8f1fb4d871..99cf001fd70f 100644
> > > > > > > --- a/xen/arch/x86/mm/mem_sharing.c
> > > > > > > +++ b/xen/arch/x86/mm/mem_sharing.c
> > > > > > > @@ -1641,6 +1641,24 @@ static void copy_vcpu_nonreg_state(struct 
> > > > > > > vcpu *d_vcpu, struct vcpu *cd_vcpu)
> > > > > > >        hvm_set_nonreg_state(cd_vcpu, &nrs);
> > > > > > >    }
> > > > > > > 
> > > > > > > +static int copy_guest_area(struct guest_area *cd_area,
> > > > > > > +                           const struct guest_area *d_area,
> > > > > > > +                           struct vcpu *cd_vcpu,
> > > > > > > +                           const struct domain *d)
> > > > > > > +{
> > > > > > > +    unsigned int offset;
> > > > > > > +
> > > > > > > +    /* Check if no area to map, or already mapped. */
> > > > > > > +    if ( !d_area->pg || cd_area->pg )
> > > > > > > +        return 0;
> > > > > > > +
> > > > > > > +    offset = PAGE_OFFSET(d_area->map);
> > > > > > > +    return map_guest_area(cd_vcpu, gfn_to_gaddr(
> > > > > > > +                                       mfn_to_gfn(d, 
> > > > > > > page_to_mfn(d_area->pg))) +
> > > > > > > +                                   offset,
> > > > > > > +                          PAGE_SIZE - offset, cd_area, NULL);
> > > > > > > +}
> > > > > > > +
> > > > > > >    static int copy_vpmu(struct vcpu *d_vcpu, struct vcpu *cd_vcpu)
> > > > > > >    {
> > > > > > >        struct vpmu_struct *d_vpmu = vcpu_vpmu(d_vcpu);
> > > > > > > @@ -1709,6 +1727,16 @@ static int copy_vcpu_settings(struct 
> > > > > > > domain *cd, const struct domain *d)
> > > > > > >                    return ret;
> > > > > > >            }
> > > > > > > 
> > > > > > > +        /* Same for the (physically registered) runstate and 
> > > > > > > time info areas. */
> > > > > > > +        ret = copy_guest_area(&cd_vcpu->runstate_guest_area,
> > > > > > > +                              &d_vcpu->runstate_guest_area, 
> > > > > > > cd_vcpu, d);
> > > > > > > +        if ( ret )
> > > > > > > +            return ret;
> > > > > > > +        ret = copy_guest_area(&cd_vcpu->arch.time_guest_area,
> > > > > > > +                              &d_vcpu->arch.time_guest_area, 
> > > > > > > cd_vcpu, d);
> > > > > > > +        if ( ret )
> > > > > > > +            return ret;
> > > > > > > +
> > > > > > >            ret = copy_vpmu(d_vcpu, cd_vcpu);
> > > > > > >            if ( ret )
> > > > > > >                return ret;
> > > > > > > @@ -1950,7 +1978,10 @@ int mem_sharing_fork_reset(struct domain 
> > > > > > > *d, bool reset_state,
> > > > > > > 
> > > > > > >     state:
> > > > > > >        if ( reset_state )
> > > > > > > +    {
> > > > > > >            rc = copy_settings(d, pd);
> > > > > > > +        /* TBD: What to do here with -ERESTART? */
> > > > > > 
> > > > > > There is no situation where we get an -ERESTART here currently. Is
> > > > > > map_guest_area expected to run into situations where it fails with
> > > > > > that rc?
> > > > > 
> > > > > Yes, there's a spin_trylock() call that will result in
> > > > > map_guest_area() returning -ERESTART.
> > > > > 
> > > > > > If yes we might need a lock in place so we can block until it
> > > > > > can succeed.
> > > > > 
> > > > > I'm not sure whether returning -ERESTART can actually happen in
> > > > > map_guest_area() for the fork case: the child domain is still paused
> > > > > at this point, so there can't be concurrent guest hypercalls that
> > > > > would also cause the domain hypercall_deadlock_mutex to be acquired.
> > > 
> > > Perhaps turning it into an ASSERT(rc != -ERESTART) is the way to go at
> > > this point. If we run into any cases where it trips we can reason it
> > > out.
> > 
> > In order to avoid possibly returning -ERESTART (which should never be
> > seen by hypercall callers) we might want to convert it to -EBUSY and
> > let the caller pick the pieces.
> 
> I realize this is a matter of taste. I think EAGAIN is a better conversion
> for ERESTART because we effectively want to caller to try again.

That's fine with me, but could we leave adding such translation to a
further patch?

I would rather modify Jans code as less as possible.

Thanks, Roger.