Re: [Xen-devel] [PATCH v2 11/25] arm/altp2m: Add HVMOP_altp2m_destroy_p2m.

[Julien Grall <julien.grall@xxxxxxx>]

On 06/08/2016 10:54, Sergej Proskurin wrote:
> Hi Julien,
Hello Sergej,

> On 08/04/2016 01:46 PM, Julien Grall wrote:
> > Hello Sergej,
> >
> > On 01/08/16 18:10, Sergej Proskurin wrote:
> > > Signed-off-by: Sergej Proskurin <proskurin@xxxxxxxxxxxxx>
> > > ---
> > > Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> > > Cc: Julien Grall <julien.grall@xxxxxxx>
> > > ---
> > > v2: Substituted the call to tlb_flush for p2m_flush_table.
> > >     Added comments.
> > >     Cosmetic fixes.
> > > ---
> > >  xen/arch/arm/altp2m.c        | 50
> > > ++++++++++++++++++++++++++++++++++++++++++++
> > >  xen/arch/arm/hvm.c           |  2 +-
> > >  xen/include/asm-arm/altp2m.h |  4 ++++
> > >  3 files changed, 55 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/xen/arch/arm/altp2m.c b/xen/arch/arm/altp2m.c
> > > index c22d2e4..80ed553 100644
> > > --- a/xen/arch/arm/altp2m.c
> > > +++ b/xen/arch/arm/altp2m.c
[...]

> > > +
> > > +            p2m_flush_table(p2m);
> > > +
> > > +            /*
> > > +             * Reset VTTBR.
> > > +             *
> > > +             * Note that VMID is not freed so that it can be reused
> > > later.
> > > +             */
> > > +            p2m->vttbr.vttbr = INVALID_VTTBR;
> > > +            d->arch.altp2m_vttbr[idx] = INVALID_VTTBR;
> > > +
> > > +            read_unlock(&p2m->lock);
> > Why did you decide to reset the p2m rather than free it? The code
> > would be simpler with the latter.
> * First, to be as similar to the x86 implementation as possible.
> * Second, simply to reuse already allocated p2m's without additional
> overhead.
I don't really buy those arguments. The x86 implementation may not fit 
the ARM model. Initializing a p2m on ARM is very quick: allocating the 
p2m and the root pages (up to 2 page).
So the overhead is really minimal. Therefore, I don't see any reason to 
introduce complex bookkeeping when NULL can be used for the same purpose.
Regards,

--
Julien Grall

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