Re: [Xen-devel] [PATCH v5 5/7] VT-d: Refactor iommu_ops .map_page() and unmap_page()

["Xu, Quan" <quan.xu@xxxxxxxxx>]

+CC Dario who is also watching this patch set.

On February 25, 2016 4:59pm, <JBeulich@xxxxxxxx> wrote:
> >>> On 25.02.16 at 07:56, <quan.xu@xxxxxxxxx> wrote:
> >> On February 17, 2016 10:23pm, <JBeulich@xxxxxxxx> wrote:
> >> >>> On 05.02.16 at 11:18, <quan.xu@xxxxxxxxx> wrote:
> >> > to pass down a flag indicating whether the lock is being held, and
> >> > check the way up the call trees.
> >>
> >> Same comments as on the previous patch; most of the changes outside
> >> of xen/drivers/passthrough/ seem to be avoidable here.
> >>
> >
> > (VT-d RMRR / P2M EPT)
> >
> > Jan,
> >    When I fix the VT-d RMRR related code
> >      1. $... iommu_map_page()/iommu_unmap_page()
> > --p2m->set_entry()--p2m_set_entry()--set_identity_p2m_entry() /
> > clear_identity_p2m_entry()-- rmrr_identity_mapping()--... ,
> >      2. $... iommu_pte_flush()
> > --p2m->set_entry()--p2m_set_entry()--set_identity_p2m_entry() /
> > clear_identity_p2m_entry()-- rmrr_identity_mapping()--... ,
> >
> >  I found that the pcidevs_lock is being held for p2m_set_entry(). It
> > is a corner case which is _not_ fix in previous patch set. As similar,
> > I think I need to add p2m_set_entry_locked() to pass down a flag
> > indicating  whether the pcidevs_lock is being held. Right?
> 
> Well, to me this would seem rather gross a hack - George, how about you? 

George, I look forward to your comments and suggestions. :)

> I'd
> really suggest investigating alternatives. One that comes to mind would be to
> move acquiring/releasing pcidevs_lock into a helper function, and setting a
> per-CPU flag indicating ownership. 

To me, this might be fine.
Does Per-CPU flag refer to this_cpu(iommu_dont_flush_iotlb) or variant?

> However, the same effect could be achieved
> by making the lock a recursive one, which would then seem to more
> conventional approach (but requiring as much code to be touched).
> Both approached would eliminate the need to pass down "locked"
> flags.
> 
> > BTW, just a quick question, what's the difference between p2m-ept.c
> > and p2m-pt.c ? Thanks.
> 
> The former, as its name says, is used for EPT, while the latter is the P2M
> implementation for both NPT and shadow mode.
>

Thanks!. Got it.


Quan

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