Re: [PATCH] arm64: mte: Use PAGE_KERNEL_TAGGED in arch_add_memory

[Catalin Marinas <catalin.marinas@arm.com>]

On Fri, Mar 05, 2021 at 04:55:28PM +0100, David Hildenbrand wrote:
> On 05.03.21 16:43, Catalin Marinas wrote:
> > On Fri, Mar 05, 2021 at 11:19:08AM +0000, Catalin Marinas wrote:
> > > On Tue, Mar 02, 2021 at 08:21:03PM -0800, Patrick Daly wrote:
> > > > In a system which supports MTE, the linear kernel region must allow
> > > > reading/writing allocation tags. For memory present at boot this
> > > > is already being done in map_mem(). Add the same in arch_add_memory().
> > > > 
> > > > Signed-off-by: Patrick Daly <pdaly@codeaurora.org>
> > > > ---
> > > >   arch/arm64/mm/mmu.c | 12 +++++++++++-
> > > >   1 file changed, 11 insertions(+), 1 deletion(-)
> > > > 
> > > > diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> > > > index 9b25d60b..0fcfe90 100644
> > > > --- a/arch/arm64/mm/mmu.c
> > > > +++ b/arch/arm64/mm/mmu.c
> > > > @@ -1463,6 +1463,7 @@ int arch_add_memory(int nid, u64 start, u64 size,
> > > >   		    struct mhp_params *params)
> > > >   {
> > > >   	int ret, flags = 0;
> > > > +	pgprot_t pgprot;
> > > >   	if (!inside_linear_region(start, size)) {
> > > >   		pr_err("[%llx %llx] is outside linear mapping region\n", start, start + size);
> > > > @@ -1477,8 +1478,17 @@ int arch_add_memory(int nid, u64 start, u64 size,
> > > >   	    IS_ENABLED(CONFIG_KFENCE))
> > > >   		flags = NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS;
> > > > +	/*
> > > > +	 * The linear map must allow allocation tags reading/writing
> > > > +	 * if MTE is present. Otherwise, it has the same attributes as
> > > > +	 * PAGE_KERNEL.
> > > > +	 */
> > > > +	pgprot = params->pgprot;
> > > > +	if (pgprot_val(pgprot) == pgprot_val(PAGE_KERNEL))
> > > > +		pgprot = PAGE_KERNEL_TAGGED;
> > > > +
> > > >   	__create_pgd_mapping(swapper_pg_dir, start, __phys_to_virt(start),
> > > > -			     size, params->pgprot, __pgd_pgtable_alloc,
> > > > +			     size, pgprot, __pgd_pgtable_alloc,
> > > >   			     flags);
[...]
> > ---------------8<-------------------------------
> > diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> > index e17b96d0e4b5..5c78b92d9ec5 100644
> > --- a/arch/arm64/include/asm/pgtable.h
> > +++ b/arch/arm64/include/asm/pgtable.h
> > @@ -486,6 +486,8 @@ static inline pmd_t pmd_mkdevmap(pmd_t pmd)
> >   	__pgprot_modify(prot, PTE_ATTRINDX_MASK, PTE_ATTRINDX(MT_NORMAL_NC) | PTE_PXN | PTE_UXN)
> >   #define pgprot_device(prot) \
> >   	__pgprot_modify(prot, PTE_ATTRINDX_MASK, PTE_ATTRINDX(MT_DEVICE_nGnRE) | PTE_PXN | PTE_UXN)
> > +#define pgprot_tagged(prot) \
> > +	__pgprot_modify(prot, PTE_ATTRINDX_MASK, PTE_ATTRINDX(MT_NORMAL_TAGGED))
> >   /*
> >    * DMA allocations for non-coherent devices use what the Arm architecture calls
> >    * "Normal non-cacheable" memory, which permits speculation, unaligned accesses
> > diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> > index cdfc4e9f253e..f5f5044db2ce 100644
> > --- a/include/linux/pgtable.h
> > +++ b/include/linux/pgtable.h
> > @@ -904,6 +904,10 @@ static inline void ptep_modify_prot_commit(struct vm_area_struct *vma,
> >   #define pgprot_device pgprot_noncached
> >   #endif
> > 
> > +#ifndef pgprot_tagged
> > +#define pgprot_tagged(prot)	(prot)
> > +#endif
> > +
> >   #ifdef CONFIG_MMU
> >   #ifndef pgprot_modify
> >   #define pgprot_modify pgprot_modify
> > diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> > index 5ba51a8bdaeb..4253d80a59ba 100644
> > --- a/mm/memory_hotplug.c
> > +++ b/mm/memory_hotplug.c
> > @@ -1072,7 +1072,7 @@ static int online_memory_block(struct memory_block *mem, void *arg)
> >    */
> >   int __ref add_memory_resource(int nid, struct resource *res, mhp_t mhp_flags)
> >   {
> > -	struct mhp_params params = { .pgprot = PAGE_KERNEL };
> > +	struct mhp_params params = { .pgprot = pgprot_tagged(PAGE_KERNEL) };
> 
> This looks like we're pushing arch specific stuff ("tagged") in here. Can't
> we generalize this to something like
> 
> pgprot_mhp_default
> 
> (or a better name)
> 
> that defaults to PAGE_KERNEL on all architectures except arm64 which
> overwrites this somehow?

It works for me but I prefer the prot modification style similar to
pgprot_writecombine() etc. (i.e. takes a parameter like PAGE_KERNEL and
just changes bits of it).

Thanks.

-- 
Catalin