Re: [Xen-devel] [PATCH RFC 04/55] x86/mm: introduce l{1, 2}t local variables to map_pages_to_xen

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Fri, Mar 15, 2019 at 09:36:37AM -0600, Jan Beulich wrote:
> >>> On 07.02.19 at 17:44, <wei.liu2@xxxxxxxxxx> wrote:
> > The pl2e and pl1e variables are heavily (ab)used in that function. It
> > is fine at the moment because all page tables are always mapped so
> > there is no need to track the life time of each variable.
> > 
> > We will soon have the requirement to map and unmap page tables. We
> > need to track the life time of each variable to avoid leakage.
> > 
> > Introduce some l{1,2}t variables with limited scope so that we can
> > track life time of pointers to xen page tables more easily.
> 
> But you retain some uses of the old variables, and to be honest it's
> not really clear to me by what criteria (and having multiple instances
> of a variable name in a single function isn't necessarily less confusing).
> I think we either stick to what's there (doesn't look too bad to me),
> or you switch to scope restricted page table pointers throughout the
> function, such that the function scope symbols can go away
> altogether).

I thought my commit message was clear enough: it helped tracking the
lifetime of pointers more easily. There is nothing new in this trick:
the less variables of the same name the better.

In this patch, places where it is clear that using local scope variables
suffices are changed to use local variables.

In the end, pl*e are only used to point to entries which point to the
page tables related to the linear address being mapped. They won't be
used to point to any intermediate pointers which are used to manipulate
page tables.

We can't eliminate function scope variables because they need to stay
function scope -- see later patches where they are unmapped in the out
path.

Wei.

> 
> Jan
> 
> 
> 
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