Re: [Xen-devel] [v2 06/11] vmx: add help functions to support PML

[Kai Huang <kai.huang@xxxxxxxxxxxxxxx>]

On 04/17/2015 02:58 PM, Jan Beulich wrote:
> > > > On 17.04.15 at 08:51, <kai.huang@xxxxxxxxxxxxxxx> wrote:
> > On 04/17/2015 02:23 PM, Jan Beulich wrote:
> > > > > > On 17.04.15 at 05:10, <kai.huang@xxxxxxxxxxxxxxx> wrote:
> > > > On 04/16/2015 11:42 PM, Jan Beulich wrote:
> > > > > > > > On 15.04.15 at 09:03, <kai.huang@xxxxxxxxxxxxxxx> wrote:
> > > > > > +void vmx_vcpu_flush_pml_buffer(struct vcpu *v)
> > > > > > +{
> > > > > > +    uint64_t *pml_buf;
> > > > > > +    unsigned long pml_idx;
> > > > > > +
> > > > > > +    ASSERT(vmx_vcpu_pml_enabled(v));
> > > > > > +
> > > > > > +    vmx_vmcs_enter(v);
> > > > > > +
> > > > > > +    __vmread(GUEST_PML_INDEX, &pml_idx);
> > > > > Don't you require the vCPU to be non-running or current when you
> > > > > get here? If so, perhaps add a respective ASSERT()?
> > > > Yes an ASSERT would be better.
> > > >
> > > > v->pause_count will be increased if vcpu is kicked out by domain_pause
> > > > explicitly, but looks the same thing won't be done if vcpu is kicked out
> > > > by PML buffer full VMEXIT. So should the ASSERT be done like below?
> > > >
> > > > ASSERT(atomic_read(&v->pause_count) || (v == current));
> > > For one I'd reverse the two parts. And then I think pause count
> > > being non-zero is not a sufficient condition - if a non-synchronous
> > > pause was issued against the vCPU it may still be running. I'd
> > > suggest !vcpu_runnable(v) && !v->is_running, possibly with the
> > > pause count check instead of the runnable one if the only
> > > permitted case where v != current requires the vCPU to be
> > > paused.
> > The vmx_vcpu_flush_pml_buffer is only supposed to be called in below cases:
> >
> >       - When PML full VMEXIT happens
> >       - In paging_log_dirty_op & hap_track_dirty_vram, before reporting
> > dirty pages to userspace.
> >       - In vmx_vcpu_disable_pml, called from vmx_vcpu_destroy, or when
> > log-dirty mode is disabled.
> >
> > In the latter two cases, domain_pause is guaranteed to be called before
> > vmx_vcpu_flush_pml_buffer is called, therefore looks there's no
> > possibility of non-synchronous pause of the vcpu.
> >
> > Or are you suggesting we should suppose this function can be called from
> > any caller, and meanwhile is able to act reasonably?
> No. All I'm saying is in order to protect against eventual undue
> future callers, it should assert that its preconditions are met. I.e.
> if the vCPU is expected to be paused, check that the pause
> count in non-zero _and_ that the pause actually took effect.
I see. I will do as you suggested:

ASSERT((v == current) || (!vcpu_runnable(v) && !v->is_running));

And v != current should be the only case requires the vcpu to be paused.

Thanks,
-Kai
> Jan
>
>
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