Re: [PATCH v4 6/7] mtrr, x86: Clean up mtrr_type_lookup()

Toshi Kani
2015-05-07T00:01:19Z

On Thu, 2015-05-07 at 00:49 +0200, Borislav Petkov wrote:
> On Wed, May 06, 2015 at 10:00:30AM -0600, Toshi Kani wrote:
> > Ingo asked me to describe this info here in his review...
>
> Ok.
>
> > mtrr_type_lookup_fixed() checks the above conditions at entry, and
> > returns immediately with TYPE_INVALID. I think it is safer to have such
> > checks in mtrr_type_lookup_fixed() in case there will be multiple
> > callers.
>
> This is not what I mean - I mean to call mtrr_type_lookup_fixed() based
> on @start and not unconditionally, like you do.
>
> And there most likely won't be multiple callers because we're phasing
> out MTRR use.
>
> And even if there are, they better look at how this function is being
> called before calling it. Which I seriously doubt - it is a static
> function which you *just* came up with.

Well, creating mtrr_type_lookup_fixed() is one of the comments I had in
the previous code review. Anyway, let me make sure if I understand your
comment correctly. Do the following changes look right to you?

1) Change the caller responsible for the condition checks.

if ((start < 0x100000) &&
(mtrr_state.have_fixed) &&
(mtrr_state.enabled & MTRR_STATE_MTRR_FIXED_ENABLED))
return mtrr_type_lookup_fixed(start, end);

2) Delete the checks with mtrr_state in mtrr_type_lookup_fixed() as they
are done by the caller. Keep the check with '(start >= 0x100000)' to
assure that the code handles the range [0xC0000 - 0xFFFFF] correctly.

static u8 mtrr_type_lookup_fixed(u64 start, u64 end)
{
int idx;

if (start >= 0x100000)
return MTRR_TYPE_INVALID;

- if (!(mtrr_state.have_fixed) ||
- !(mtrr_state.enabled & MTRR_STATE_MTRR_FIXED_ENABLED))
- return MTRR_TYPE_INVALID;

> > Right, and there is more. As the original code had comment "Just return
> > the type as per start", which I noticed that I had accidentally removed,
> > the code only returns the type of the start address. The fixed ranges
> > have multiple entries with different types. Hence, a given range may
> > overlap with multiple fixed entries. I will restore the comment in the
> > function header to clarify this limitation.
>
> Ok, let's cleanup this function first and then consider fixing other
> possible bugs which haven't been fixed since forever. Again, we might
> not even need to address them because we won't be using MTRRs once we
> switch to PAT completely, which is what Luis is working on.

Right.

Thanks,
-Toshi