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

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

On Mon, Mar 08, 2021 at 12:00:17PM +0100, David Hildenbrand wrote:
> On 05.03.21 18:44, Catalin Marinas wrote:
> > 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).
> 
> Sure, I'm wondering if we can abstract the "tagged" part here.
> 
> pgprot_mhp(PAGE_KERNEL) whatsoever.
> 
> But maybe we do consider the "tagged" part okay in common code already (it
> felt quite arm64 specific to me at first). I can spot untagged_addr() all
> over the place. So maybe pgprot_tagged() is just fine.

That was my reasoning initially though 'tagged' in untagged_addr()
refers to some bits in the top part of the address and that will become
more common. 'Tagged' as a mapping attribute is specific to arm64 and
sparc for now.

That said, pgprot_mhp() is probably better as the core code cannot tell
whether the arch implementation wants the hotplugged memory tagged or
not. Similarly, we'll add a pgprot_vmalloc() at some point.

-- 
Catalin