[PATCH 20/35] arm64: mte: Add in-kernel MTE helpers

[Andrey Konovalov]

On Thu, Aug 27, 2020 at 11:38 AM Catalin Marinas
<catalin.marinas at arm.com> wrote:
>
> On Fri, Aug 14, 2020 at 07:27:02PM +0200, Andrey Konovalov wrote:
> > diff --git a/arch/arm64/include/asm/mte.h b/arch/arm64/include/asm/mte.h
> > index 1c99fcadb58c..733be1cb5c95 100644
> > --- a/arch/arm64/include/asm/mte.h
> > +++ b/arch/arm64/include/asm/mte.h
> > @@ -5,14 +5,19 @@
> >  #ifndef __ASM_MTE_H
> >  #define __ASM_MTE_H
> >
> > -#define MTE_GRANULE_SIZE     UL(16)
> > +#include <asm/mte_asm.h>
>
> So the reason for this move is to include it in asm/cache.h. Fine by
> me but...
>
> >  #define MTE_GRANULE_MASK     (~(MTE_GRANULE_SIZE - 1))
> >  #define MTE_TAG_SHIFT                56
> >  #define MTE_TAG_SIZE         4
> > +#define MTE_TAG_MASK         GENMASK((MTE_TAG_SHIFT + (MTE_TAG_SIZE - 1)), MTE_TAG_SHIFT)
> > +#define MTE_TAG_MAX          (MTE_TAG_MASK >> MTE_TAG_SHIFT)
>
> ... I'd rather move all these definitions in a file with a more
> meaningful name like mte-def.h. The _asm implies being meant for .S
> files inclusion which isn't the case.
>
> > diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
> > index eb39504e390a..e2d708b4583d 100644
> > --- a/arch/arm64/kernel/mte.c
> > +++ b/arch/arm64/kernel/mte.c
> > @@ -72,6 +74,47 @@ int memcmp_pages(struct page *page1, struct page *page2)
> >       return ret;
> >  }
> >
> > +u8 mte_get_mem_tag(void *addr)
> > +{
> > +     if (system_supports_mte())
> > +             addr = mte_assign_valid_ptr_tag(addr);
>
> The mte_assign_valid_ptr_tag() is slightly misleading. All it does is
> read the allocation tag from memory.
>
> I also think this should be inline asm, possibly using alternatives.
> It's just an LDG instruction (and it saves us from having to invent a
> better function name).

Could you point me to an example of inline asm with alternatives if
there's any? I see alternative_if and other similar macros used in
arch/arm64/ code, is that what you mean? Those seem to always use
static conditions, like config values, but here we have a dynamic
system_supports_mte(). Could you elaborate on how I should implement
this?