Re: [PATCH v2] x86/hvm/viridian: fix the TLB flush hypercall

[Paul Durrant <Paul.Durrant@citrix.com>]

> -----Original Message-----
> From: Jan Beulich [mailto:JBeulich@suse.com]
> Sent: 16 March 2016 15:36
> To: Paul Durrant
> Cc: Andrew Cooper; xen-devel@lists.xenproject.org; Keir (Xen.org)
> Subject: Re: [PATCH v2] x86/hvm/viridian: fix the TLB flush hypercall
> 
> >>> On 16.03.16 at 15:21, <paul.durrant@citrix.com> wrote:
> > v2:
> >  - Move to per-pcpu ipi mask.
> >  - Use smp_send_event_check_mask() to IPI rather than flush_tlb_mask().
> > ---
> >  xen/arch/x86/hvm/hvm.c             | 12 ------------
> >  xen/arch/x86/hvm/viridian.c        | 19 ++++++-------------
> >  xen/include/asm-x86/hvm/viridian.h |  4 ----
> >  3 files changed, 6 insertions(+), 29 deletions(-)
> 
> Quite nice for a bug fix.
> 
> > @@ -656,7 +647,9 @@ int viridian_hypercall(struct cpu_user_regs *regs)
> >           * so we may unnecessarily IPI some CPUs.
> >           */
> >          if ( !cpumask_empty(pcpu_mask) )
> > -            flush_tlb_mask(pcpu_mask);
> > +            smp_send_event_check_mask(pcpu_mask);
> > +
> > +        output.rep_complete = input.rep_count;
> 
> Questions on this one remain: Why only for this hypercall? And
> what does "repeat count" mean in this context?
> 

It's only for this hypercall because it's the only 'rep' hypercall we implement. For non-rep hypercalls the spec states that the rep count and starting index in the input params must be zero. It does not state what the value of reps complete should be on output for non-rep hypercalls but I think it's safe to assume that zero is correct.
For rep hypercalls the spec says that on output "the reps complete field is the total number of reps complete and not relative to the rep start index. For example, if the caller specified a rep start index of 5, and a rep count of 10, the reps complete field would indicate 10 upon successful completion".

Section 12.4.3 of the spec defines the HvFlushVirtualAddressList hypercall as a rep hypercall and each rep refers to flush of a single guest VA range. Because we invalidate all VA ranges in one go clearly we complete all reps straight away :-)

  Paul

> Jan


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