[RFC 1/2] powerpc/swiotlb: Dont free up allocated SWIOTLB slab on POWER

[Ram Pai]

On Wed, Apr 04, 2018 at 10:48:31PM +1000, Michael Ellerman wrote:
> Anshuman Khandual <khandual at linux.vnet.ibm.com> writes:
> > Even though SWIOTLB slab gets allocated and initialized on powerpc with
> > swiotlb_init() called during mem_init(), it gets released away again on
> > POWER platform because 'ppc_swiotlb_enable' never gets set. The function
> > swiotlb_detect_4g() checks for 4GB memory and then sets the variable
> > 'ppc_swiotlb_enable' which prevents freeing up the SWIOTLB slab. Lets
> > make POWER platform call swiotlb_detect_4g() during setup_arch() which
> > will keep the SWIOTLB slab through out the runtime.
> >
> > A previous commit cf5621032f ("powerpc/64: Limit ZONE_DMA32 to 4GiB in
> > swiotlb_detect_4g()") enforced 4GB limit on ZONE_DMA32 which is is not
> > applicable on POWER (CONFIG_PPC_BOOK3S_64) platform. Lets remove this
> > unnecessary restriction.
> 
> You're using "POWER" to mean Book3S 64-bit, but "POWER" is something
> else (the ISA for POWER1/POWER2).
> 
> So please just say "Book3S 64-bit" or talk about a specific CPU, eg.
> Power9.
> 
> > After the patch, SWIOTLB slab does not get released.
> >
> > [0.410992] software IO TLB [mem 0xfbff0000-0xffff0000] (64MB) mapped
> > at [00000000767f6cb3-000000004a10114f]
> 
> But we don't want SWIOTLB on any existing Book3S 64-bit platforms, so
> leaving it enabled is just wasting memory.
> 
> > diff --git a/arch/powerpc/kernel/setup-common.c b/arch/powerpc/kernel/setup-common.c
> > index d73ec518ef80..c4db844e0b0d 100644
> > --- a/arch/powerpc/kernel/setup-common.c
> > +++ b/arch/powerpc/kernel/setup-common.c
> > @@ -944,6 +944,7 @@ void __init setup_arch(char **cmdline_p)
> >  	/* Initialize the MMU context management stuff. */
> >  	mmu_context_init();
> >  
> > +	swiotlb_detect_4g();
> 
> You shouldn't be calling this, your use case has nothing to do with 4GB.
> 
> Instead when you detect that you're running under the ultravisor then
> you should set ppc_swiotlb_enable = 1.

Please assume that you will have a interface to detect if you are
running in a protected-environment(AKA secure-environment).

So maybe you can rename swiotlb_detect_4g() to swiotlb_detect() 
and modify that function to enable or disable ppc_swiotlb_enable
depending on the availability of 4g or availability of 
protected-environment?

RP