Re: [PATCH] armv8: mmu: don't switch to emergency tlb when adding a dynamic mapping

[Marc Zyngier <maz@kernel.org>]

On Wed, 08 Jan 2025 15:19:07 +0000,
Caleb Connolly <caleb.connolly@linaro.org> wrote:
> 
> Hi Marc,
> 
> Thanks for your comments.
> 
> On 08/01/2025 16:05, Marc Zyngier wrote:
> > On Wed, 08 Jan 2025 14:22:24 +0000,
> > Caleb Connolly <caleb.connolly@linaro.org> wrote:
> >>
> >> This seems to cause crashes on a bunch of Qualcomm platforms. It's safer
> >> to just update the live table and flush it.
> > 
> > You may want to provide a bit more information, because that's not
> > much to go on, really.
> 
> Best I have is "the board hangs and then resets when trying to switch
> pagetables", I didn't manage to narrow it down exactly, but since it
> seems to work to update the tables without switching that feels like a
> better approach at least for now.

I think you should completely characterise the issue before changing
things like this. My hunch is that the so-called "emergency" page
tables do not map the page tables you're modifying. Or something along
those lines. But you really want to get to the bottom of it.

> 
> In hindsight, this mmu_map_region() function is not great in a lot of
> ways, I'm still getting my head around the MMU and I expect this will
> keep being improved.
> > 
> >>
> >> Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>
> >> ---
> >>  arch/arm/cpu/armv8/cache_v8.c | 11 ++---------
> >>  arch/arm/include/asm/system.h |  3 +--
> >>  drivers/soc/qcom/cmd-db.c     |  2 +-
> >>  3 files changed, 4 insertions(+), 12 deletions(-)
> >>
> >> diff --git a/arch/arm/cpu/armv8/cache_v8.c b/arch/arm/cpu/armv8/cache_v8.c
> >> index e6be6359c5d9..43051d156122 100644
> >> --- a/arch/arm/cpu/armv8/cache_v8.c
> >> +++ b/arch/arm/cpu/armv8/cache_v8.c
> >> @@ -338,9 +338,9 @@ static void map_range(u64 virt, u64 phys, u64 size, int level,
> >>  		size -= next_size;
> >>  	}
> >>  }
> >>  
> >> -void mmu_map_region(phys_addr_t addr, u64 size, bool emergency)
> >> +void mmu_map_region(phys_addr_t addr, u64 size)
> >>  {
> >>  	u64 va_bits;
> >>  	int level = 0;
> >>  	u64 attrs = PTE_BLOCK_MEMTYPE(MT_NORMAL) | PTE_BLOCK_INNER_SHARE;
> >> @@ -350,19 +350,12 @@ void mmu_map_region(phys_addr_t addr, u64 size, bool emergency)
> >>  	get_tcr(NULL, &va_bits);
> >>  	if (va_bits < 39)
> >>  		level = 1;
> >>  
> >> -	if (emergency)
> >> -		map_range(addr, addr, size, level,
> >> -			  (u64 *)gd->arch.tlb_emerg, attrs);
> >> -
> >> -	/* Switch pagetables while we update the primary one */
> >> -	__asm_switch_ttbr(gd->arch.tlb_emerg);
> >> -
> >>  	map_range(addr, addr, size, level,
> >>  		  (u64 *)gd->arch.tlb_addr, attrs);
> >>  
> >> -	__asm_switch_ttbr(gd->arch.tlb_addr);
> >> +	flush_dcache_range(gd->arch.tlb_addr, gd->arch.tlb_size);
> > 
> > Why would you invalidate anything when *mapping* something? By
> > definition, if there was nothing mapped before, there is nothing to
> > invalidate (hint: the architecture forbids negative caching).
> 
> This isn't flushing the region or TLB, it's flushing the translation
> table we just modified.

Ah, you're obviously right. I can't read today...

But it then begs the question: are the page tables configured to be
accessed by the page-table walker in a non-cacheable fashion? That'd
be rather silly, and the only reason why you'd need any form of CMO at
this point.

> I'm wondering if this tlb_addr variable is misnamed, I'm reading ARM
> docs which suggest the TLB is some cache inside(?) the MMU, but this
> tlb_addr in U-Boot actually points to the translation table in memory if
> my understanding is correct?

That's my impression, but each time I look at this code, I feel a bit
sick...

	M.

-- 
Without deviation from the norm, progress is not possible.