Re: [PATCH v6 26/34] iommu/amd: Allow the AMD IOMMU to work with memory encryption

[Tom Lendacky]

On 6/15/2017 10:33 AM, Borislav Petkov wrote:
> On Thu, Jun 15, 2017 at 09:59:45AM -0500, Tom Lendacky wrote:
> > Actually the detection routine, amd_iommu_detect(), is part of the
> > IOMMU_INIT_FINISH macro support which is called early through mm_init()
> > from start_kernel() and that routine is called before init_amd().
>
> Ah, we do that there too:
>
> 	for (p = __iommu_table; p < __iommu_table_end; p++) {
>
> Can't say that that code with the special section and whatnot is
> obvious. :-\
>
> Oh, well, early_init_amd() then. That is called in
> start_kernel->setup_arch->early_cpu_init and thus before mm_init().
>
> > > If so, it did work fine until now, without the volatile. Why is it
> > > needed now, all of a sudden?
> >
> > If you run checkpatch against the whole amd_iommu.c file you'll see that
>
> I'm, of course, not talking about the signature change: I'm *actually*
> questioning the need to make this argument volatile, all of a sudden.

Understood.

> If there's a need, please explain why. It worked fine until now. If it
> didn't, we would've seen it.

The original reason for the change was to try and make the use of
iommu_virt_to_phys() straight forward.  Removing the cast and changing
build_completion_wait() to take a u64 * (without volatile) resulted in a
warning because cmd_sem is defined in the amd_iommu struct as volatile,
which required a cast on the call to iommu_virt_to_phys() anyway. Since
it worked fine previously and the whole volatile thing is beyond the
scope of this patchset, I'll change back to the original method of how
the function was called.

> If it is a bug, then it needs a proper explanation, a *separate* patch
> and so on. But not like now, a drive-by change in an IOMMU enablement
> patch.
>
> If it is wrong, then wait_on_sem() needs to be fixed too. AFAICT,
> wait_on_sem() gets called in both cases with interrupts disabled, while
> holding a lock so I'd like to pls know why, even in that case, does this
> variable need to be volatile

Changing the signature back reverts to the original way, so this can be
looked at separate from this patchset then.

Thanks,
Tom

>