RE: [PATCH v2] x86/svm: do not try to handle recalc NPT faults immediately

[Paul Durrant <xadimgnik@xxxxxxxxx>]

> -----Original Message-----
> From: Jan Beulich <jbeulich@xxxxxxxx>
> Sent: 03 June 2020 11:03
> To: Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>
> Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx; andrew.cooper3@xxxxxxxxxx; wl@xxxxxxx; 
> roger.pau@xxxxxxxxxx;
> george.dunlap@xxxxxxxxxx; Paul Durrant <paul@xxxxxxx>
> Subject: Re: [PATCH v2] x86/svm: do not try to handle recalc NPT faults 
> immediately
> 
> On 02.06.2020 18:56, Igor Druzhinin wrote:
> > A recalculation NPT fault doesn't always require additional handling
> > in hvm_hap_nested_page_fault(), moreover in general case if there is no
> > explicit handling done there - the fault is wrongly considered fatal.
> >
> > This covers a specific case of migration with vGPU assigned on AMD:
> > at a moment log-dirty is enabled globally, recalculation is requested
> > for the whole guest memory including directly mapped MMIO regions of vGPU
> > which causes a page fault being raised at the first access to those;
> > but due to MMIO P2M type not having any explicit handling in
> > hvm_hap_nested_page_fault() a domain is erroneously crashed with unhandled
> > SVM violation.
> >
> > Instead of trying to be opportunistic - use safer approach and handle
> > P2M recalculation in a separate NPT fault by attempting to retry after
> > making the necessary adjustments. This is aligned with Intel behavior
> > where there are separate VMEXITs for recalculation and EPT violations
> > (faults) and only faults are handled in hvm_hap_nested_page_fault().
> > Do it by also unifying do_recalc return code with Intel implementation
> > where returning 1 means P2M was actually changed.
> >
> > Since there was no case previously where p2m_pt_handle_deferred_changes()
> > could return a positive value - it's safe to replace ">= 0" with just "== 0"
> > in VMEXIT_NPF handler. finish_type_change() is also not affected by the
> > change as being able to deal with >0 return value of p2m->recalc from
> > EPT implementation.
> >
> > Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> > Signed-off-by: Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>
> 
> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
> albeit preferably with ...
> 
> > @@ -448,12 +451,14 @@ static int do_recalc(struct p2m_domain *p2m, unsigned 
> > long gfn)
> >              clear_recalc(l1, e);
> >          err = p2m->write_p2m_entry(p2m, gfn, pent, e, level + 1);
> >          ASSERT(!err);
> > +
> > +        recalc_done = true;
> >      }
> >
> >   out:
> >      unmap_domain_page(table);
> >
> > -    return err;
> > +    return err ?: (recalc_done ? 1 : 0);
> 
> ... this shrunk to
> 
>     return err ?: recalc_done;
> 
> (easily doable while committing).
> 
> Also Cc Paul.
> 

paging_log_dirty_enable() still fails global enable if has_arch_pdevs() is 
true, so presumably there's no desperate need for this to go in 4.14?

  Paul

> Jan