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

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

On Tue, Sep 08, 2020 at 03:23:20PM +0200, Andrey Konovalov wrote:
> On Thu, Aug 27, 2020 at 11:38 AM Catalin Marinas
> <catalin.marinas@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?

There are plenty of ALTERNATIVE macro uses under arch/arm64, see
arch/arm64/include/asm/alternative.h for the definition and some simple
documentation.

In this case, something like (untested, haven't even checked whether it
matches the mte_assign_valid_ptr_tag() code):

	asm(ALTERNATIVE("orr %0, %1, #0xff << 56", "ldg %0, [%1]", ARM64_HAS_MTE));

-- 
Catalin

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